Water evaporates in hell. Also what stops you from just flooding hell another way like duplicating honey?I understand the other points you made, but this? Realistically speaking: if there were to be an Underground Ocean biome, what would it be? Cus if you ask me, it would all be underwater. Underwater caverns. And while that is not necessarily a bad idea, the problem would be to make sure that all that water doesn't pour out into hell. That would create empty, boring caves at the bottom.
Also if you claim underground ocean doesn't make sense I have one point to bring up. Does an underground jungle make sense?