Allows you to cast Divine Spells Underwater.
This won't do for water domain simply because Water Domain is more of an "Ice' Domain for me.
While this is a domain-related suggestion it would not involve my 2DA work, except for referencing the correct feat which should not be difficult for someone like Snoteye.
It is feasible and not too difficult. Whether or not allowing Sea Cave clerics to cast underwater would be overpowered is the domain of more active and balance-minded DMs.
I'm not sure how this has anything to do with 2DAs at all. Don't we handle underwater casting strictly through scripting?
Perhaps instead reduce the spell-phail pentalty consideribly for Sea Cave Domain spells and mayhaps 0-2nd circle spells? Not too powerful spells, but it'll be an edge for being familiar with the sea and underwater than a cleric who doesn't.
'
It'd make sense that an Umberlee cleric can cast at least -some- spells underwater (at the very least with more ease) that say, a Tormite or Illmatari.
You'd just need to know the index of the feat granted by the Sea Caves domain. Which might be a constant in an include, or listed in some post somewhere. But failing those sources, it's in the 2DA, too.