Underwater cave on island with Eldberg Lighthouse -- any quests associated with it?

[JMO]

I was heading up to the lighthouse and came to that group of bandits. Got the treasure hunt quest "Pearls of the Coast". I've done this part before.

While swimming underwater looking for the quest's treasurer, I noticed an underwater cave. Went in there, killed a few drowners, but was it. I didn't see any treasure in there and don't remember ever being in there in previous playthroughs.

Almost every cave in this game is associated with... something. Some side quest, side objective, etc. But not this one as far as I can see.

Does this cave play a role in some other quest I've never hit before?

[Kennedy]

It's connected to a contract.

[JMO]

Bummer. I appear to keep missing that contract whatever it is. This is my 3rd playthrough. Oh well.

[Protoman]

Some contracts have a tiny window when they're available, and there was at least one that only showed up at night (I think the dwarf near one of the exits of Novigrad?)

[RezoInverse]

You can go there for a main mission, at the end of the game.
But only if you make some specific error/choice Ask for help finding the Sunstone, but ignore the guy looking for the legendary whale. You will need another solution and that solution will lead you to this cave. .