Idk How this thing will work out but a feature, player to go underwater and explore the underwater world fight underwater mobs, find hidden caves with chests, mini-boss/boss or bounties, and all, will be the next level feeling and it will add more depth into the world
some suggestions on how it will work-
using special runes/potions/enchantments to breathe underwater
what else can be found underwater -
high lvl resources/herbs to make potions, armor, and all
sunken shipwrecks with loot.