Its hard enough to run sea-borne adventures in RPGs, let alone under water adventures.
Years ago we published The Hermit by Gary Gygax; it contained a really cool sequence where shrunken characters found themselves at the bottom of a pond, looking up at swimming fish. It opened up a whole new vista of adventuring material, material usually unreachable by most game masters.
But cracking that nut offers wonderful potential. When one reads about the giant squid, that grows up to 40+ feet long, it is easy to imagine the chaos of such a battle. A creature moving in all the space around you, above and below, behind and in front; tentacles grasping and pulling you in. Its a true monster of the deep. And this does even take into account the other actual creatures that dwell in the earth's oceans.
Very cool stuff, if it can be cracked.