It would be great if treasure dropped in the ocean or sunk with ships stayed on the sea floor when it sinks in seas shallow enough to swim down instead of despawning(at least for an hour or so). I also think some treasure for treasure quests should be underwater in reefs, wrecks and caves. And maybe some merchant quests for special fish or something?
It would encourage treasure diving which could be fun. Especially as more sea life is added. Threats like squid, sharks, eels and baracuda. And beautiful things like dolphins and schools of fish could surround lovely underwater environments.
It would also encourage more underwater landscapes in general, maybe even air pocket caves and wrecks with entrances under the sea.