Single target damage is probably the one area that casters don't natively have great answers (though polymorph and the summon spells are certainly *good* answers, and also why I don't think they're good for the game). In those situations, as the sorc I would look to land the debuff, slow. Or if I took haste, I'd slap that on our best striker. And then support with barbs, vortex warp, etc.

Underwater IME is rare enough that I'm not particularly worried about having an answer on my daily list. If it comes up, well, it's not like the rogue is any better or worse off (though, slow still works fine underwater! As does any spell that isn't fire). It's the kind of thing that warrants having a helm of underwater breathing in your backpack, and not necessarily spending a slot on it, especially as a sorcerer.

Note too that I do have two selections left. I would those two based on the campaign. If I knew there was lots of intrigue, detect thoughts and an illusion like disguise self would be good picks.