Sword should be spear, also, glowstone should look like a lamp with a metal cage around it or something like that. Zombies could be... lobster? Stone should look kind of jagged, like ou see in reefs. The mushrooms could be different types of coral. For example a brown full grown mushroom top could look like brain coral. Cobble, should be old/medieval looking. You should also change chaim mail to chains because they do have chain mail diving gear. Just a few suggestions :) Cool project though

Underwater based Will grass look like coral? that sounds pretty cool.

i think the word your looking for is bulk head

can u add more pics