At the risk of over-using a meme. Double table flip on this one.

Level 18, went exploring underwater ruin. Grabbed a mace under some stairs, turned around and a slaughterfish was ankle biting me, taking about a pixel off the health meter each time.

Cute, but I couldn't get past him. No way to fight under water, couldn't cast spells and can't push a fish out of the way.

Drowned...

DiD play, but this is testing the bounds. I'm trying to reason whether this is anything but a fail of the game mechanics/physics.

Open console, click on fish, enter the following console commands:

killdisablemarkfordelete

That last one really is for the catharsis through metaphysical annihilation of the fish. I have a batch file and use it on the Shor-damned bears all the time.

That's a shame. Sounds like a fault on your lack of awareness & Bethesda's lack of foresight. It could go either way. Really, you should just decide whether you're up to start another character or not. It's your time & your game.