Beast of the Sea: Totally Unusable

[Luccan]

Why are so many DMs putting random air-filled caves underwater? That seems like an odd feature for an underwater cave.

Underwater adventuring generally isn't particularly well thought out and few parties can do it well regardless. You need to cast a spell every day for it to work for the vast majority of PCs for longer adventures and it interacts with fighting in a way a lot of players won't find fun (both explicitly by RAW and by the DM answering questions like "what happens if you use Lightning Bolt at the bottom of the ocean?"). And if you're too far below the surface you absolutely need Darkvision to see anything and the human Fighter probably doesn't have the Light cantrip. Breathable caves might not make much sense, but you can at least try to eat your cake and have it too by doing the one big watery fight outside or in the main chamber at the end, while the rest of it takes place in a fairly normal environment. The sort of place your human Fighter can use a flint and steel and torches.

Depending on the bad guys, they might also be able to breath air and a fairly dry cavern, if they could manage it, would also be ideal for keeping certain treasures whole for a longer period of time.

[PhoenixPhyre]

I'm in the middle of an underwater arc. Don't tell my players, but not all of it will be wet. Why? Because magic. And kneecaps. Swimming things don't have kneecaps very often, and one player gets really grumpy if they can't break kneecaps every few sessions.

[Yakk]

Why are so many DMs putting random air-filled caves underwater? That seems like an odd feature for an underwater cave.

Have you never played terraria?