He can use his Quirk underwater, but it probably won't go very far and mostly would just end up heating the water around him or providing a bit of light. He's fine in the rain, but underwater's kinda not his forte, hah. Oh that'd be fun. "Look! A map of ancient times!" vs "Look! A chest of coins!" Not that "money" is of much value here.

Dabi's unfortunately aware that Nara'a no pushover, so he wouldn't opt to attack him on a normal day. He's wary about people with "fireproof" things, like armor or spells or whatever. But in this case, since he's under the influence, yeah he'd probably try to get it. But lbr, they've got a fight between them coming down the pipe at some point.