RYA has a Case RYA2017-01 (appeal) on it (pg 165 of the PdF)

Summary: "At a mark laid adjacent to a continuing obstruction, the obligation of outside boats to give room to pass the continuing obstruction continues to apply. There is no requirement for boats to give mark-room to inside boats at the mark, who may only pass the mark on the required side while giving room for the continuing obstruction and, if windward boats, keeping clear. "

Later in the Q&A, they ask if the wind was from the left (OP) instead of the right (RYA case) .. and the answer is that it's the same. 

Greg,

As a practical matter, this sort of pin-wheel at a mark this close to shore is certainly something that the Race Committee would like to avoid. But, it happens. Angelo has given you the definitive answer. 

It may be helpful to consider that avoiding a collision with an obstruction will generally over-rule whatever rights another competitor had under some other rule - 18 in this case. Safety trumps tactical advantage in almost every case I can think of.

The last sentence of 19.1 makes it clear when it says "However, at a continuing obstruction, rule 19 always applies and rule 18 does not."

questions generated from the drawing:
1. Which boats made contact?  (Was there contact between blue and orange?  Between Orange and turquoise? Between Turquoise and Purple?
3. type of boat?
4. Speed?
5 How much room is there between boats? the drawing looks like about a 1/4 of a BL?
6. when did the overlap begin?  before the continuing obstruction or after? 

This Case used to be my absolute favourite.
Under an earlier interpretation there was the possibility to disqualify everyone involved!

All,

Thanks for the reference to the case and also the clarification of how 19 overrules 18 in this case.   Nice and clear!

Paul,

I believe contact was in the middle of the pack, who had no reasonable way to avoid contact as they were squeezed from both sides.
Mixed fleet of everything from melges 20 to 50 footers.  Several divisions all arrived at the same mark at the same time.
Moderate speed in 10-15kn TWS
Gaps were initially about 1/2 boat length as booms were out and some poles were up.  That got squeezed pretty quickly. 
The overlap began long before the mark and there were boats in front and behind, with everybody overlapping each other.

Ultimately I think a key issue was the fact that the boats on the mark were unaware that on the other side of the pack there was a boat needing water.   The hail was not passed on and I think they assumed it was just usual argy bargy as opposed to an actual call for water.

Greg,
What is a call for water and how does it affect any boat's obligations or entitlements?

John,

I don't mean to imply that the hail for "water" is in any way affecting obligations or entitlements.  In this case it was simply a larger boat with a deeper draft informing the boats outside her that she needed the room she was entitled to by 19.  She has no need to hail anything to have that entitlement, but it is still a common practise as what is an obstruction for one boat may not be an obstruction for another.  So it was an informative hail, not a required one.

If the boat calling water had simple come down without hailing, then the boats outside her (at the continuing obstruction) would still owe her room.  Likewise, a hail of  "up" from a leeward boat to a windward boat is not required, but often made to inform them that a entitlement is being taken.     In this case, if the boat didn't call water, she would probably been hailed "up", which is essentially what happened once the need for room had been communicated over several boats.

I'm very clear that it is only 20 that has a required hail.  But I'm also very clear that hailing your intentions to inform other boats your thoughts on a situation can also be good practise ("I have an obstruction, I have to come down", "there is no overlap", "I'm coming up", "there is no room there" etc.)

"What is a call for water and how does it affect any boat's obligations or entitlements?"

Yea ... here we have "water" again. We had a really long discussion where people said "water" was universally understood where commonly used to mean "room to tack" ... but now it means room to pass an obstruction. 

Agree with Ang here. People ought not to say random nouns that they happen to be SURROUNDED BY if they want a much more complex thought to be understood.

Not only is it functionally irrelevant, it's setting everyone up for a foul or collision based solely on misunderstanding.

Niko and Ang,  'water' is not a random noun.  It is a well known hail throughout the British Empire.  See RYA Appeals 1968/11, 1982/6, 2003/1 and 2005/15.

What the heck RYA!?!?!? Why does that word mean so many things? From those appeals, the term clearly means "room" and yet we're allowing them to use a two-syllable word with unclear meaning when a monosyllabic one that is DEFINED IN THE RULES already exists. No wonder we're hemorrhaging participants!

When I started sailing in the UK in the 70s "water" was the standard hail for room at a mark. I don't know I can recall people hailing for room.  So UK appeals from those sort of dates would almost inevitably have people hailing "water".

Well MHYC t/light race
Has everyone missed the main point here Div 1 was not changing course at the mark just having to pass it on the STB hand and continue STH.
Other divisions where rounding and heading back NTH,
We had the outside boat calling Starboard on us a vessel clear behind,smaller normally slower vessel catching us because of the wind blanket and not avoiding a collision missed by 5mm after we had to change course to miss.Absolute chaos 
We were the most inside vessel and ahead I did not hear a single call for room between any of the other vessels behind to each other.
Why are we even talking about bout room and continuous obstructions here the main point this was a dangerous situation and should never happen again.

I was one of the skippers in the middle of the melee. While clearly the main point in all this is the supremacy of Rule 19, some of my thoughts may be interesting.
There were more boats than drawn in Greg's initial diagram.
All the boats were on port, until later in the incident when some larger boats up near the shore gybed to starboard.
There was at least one contact. My boat was "hit' from astern, but it was more a kiss than a collision (no damage). Neither skipper had any chance of avoiding the collision; there was nowhere to go.
The Nielsen mark was a mark of the course for Divs 3 and 4, but was not a mark for Divs 1 and 2. While this makes no difference re rule 19, it certainly contributed to the congestion, and to the actions of some of the Div 3 or 4 boats. I guess that none of the Divs 3 and 4 boats realized that Divs 1 and 2 were not rounding Nielsen Park. To my understanding Divs 1 and 2 were not subject to Rule 18 as they were not at mark, but the Div 3 and 4 skippers likely thought they had mark room rights.
The call for room (or water) WAS passed along. The Div 3 or 4 boats below me were coming up in order to lay their mark. I was calling the boats below me, as clearly and loudly as my untrained voice allowed, that there was an obstruction and they needed to bear away. But that communication did not get through and they continued to come up. Communication between boats is always difficult and the skippers were no doubt focused on laying their mark.
Maybe we should have gybed onto starboard to make our rights clearer, but likely our boom would have hit another boat, and anyway it would not have helped if the Div 3 or 4 boats thought they were entitled to mark room (more than 90 deg off the wind).
So for me the learnings might be:
For race organizations: - the Nielsen Park mark is too close to the shore and should be moved out (no doubt easier said than done, but...)
                                      - avoid designing courses that have some divisions rounding a mark that is not a mark for other divisions but may lie on their rhum line
Skippers: - It would have been better tactics for Div 1 and 2 boats to have anticipated the melee, and gybed away to pass Nielsen Park on the leeward (harbour) side
                - when sailing near an obstruction, you need to keep a very close eye on boats that are, or are likely to become, between you and the obstruction, and take early action to give room.
                - remember Rule 19's priority

I accidently hit "submit"
Continuing
- I could not see the actions of the larger, faster, boats that caught up with the melee and chose to go above the other boats, close to the shore, but based on discussions with other skippers it is not clear to me that they complied with Rule 19.2 (c). Certainly, in the end they did not have room, and tactically would have been well advised to gybe out and pass the other boats to leeward.

Correction: Nielsen Park WAS a mark for Div 1.
So we had for Nielsen mark
Div 1 : passing mark
Div 2 Not a mark of the course
Divs 3 and 4 rounding mark
So my comment that Div 1 boats could have gybed out and gone to leeward of the Nielsen mark was wrong.
I think my other remarks remain valid.

The U.S. Ambassador to the U.K. title character on the Netflix series "The Diplomat" loudly hailed "Water!" as a needed distraction in a crowded room, instead of dangerously hailing "Fire!" 
Worked a charm.
Clearly a U.K. thing.