One of the things I appreciate about Dungeons and Dragons Fifth Edition over any previous edition of D&D is the simplicity of their rules regarding underwater adventuring. Trying to fight something underwater? It’s either impossible or you’re bad at it. Unless you’re used to being underwater due to exposure or training, have magical aid, or are a type of person who just lives under water. In previous versions, there were a lot of rules about the types of actions that you can take under water, how shooting things works, how to attack things when the attacker or the attackee is out of the water and the other one is in it, and then all the fiddly little compounding numbers that come out of those declarations and determinations.