For some reason, many people dread taking their adventures under the Seas...or hell, even into a Lake. It IS hard at times because the rules become much more complex when you are dealing with swimming, drowning, several planes of combat in a spherical area around each character...on and on and on. The thing is, few people have trouble with Flying Creatures and gaming "In the Sky" right? So what gives?
In my own experience, I think it has a great deal to do with breathing and weight...and rust! Rust is the worst because it destroys all those cool magical swords and armor. Oh, wait, magical items don't rust...so that one is knocked out immediately! So lets get on to the ones that really bother players and DMs alike.
BREATHING: Unless the thief, fighter and so on have someone with them who can cast spells that enable them to breathe under water, adventurers are scared of drowning! Even at higher levels, characters without spells can't breathe under the waves without someone else or a magical item. Then there is the thing of "But if I give everyone a magical device to breathe water, it just seems to cheapen the uniqueness of the magical item!" That is true. So what do you do?
Monsters! Yep, Monsters are the key to helping you delve beneath the waves. There are critters to help you get this done. Using a particular type of monster in an "Adventure Hook" (a way to snare the party into thinking it is a cool idea to go for a long walk on a short pier) you can have your characters wet in no time.
Lizardmen: These folk usually don't mix well with other races, but there is no reason they can't. In fact, they sometimes are mercenaries in armies; so it wouldn't be out of the question for a party to eventually get a Lizardman as a cool Henchman...or even a full party member; but usually only from among "Advanced" Lizardmen. There is NOTHING that prevents a party from visiting this Lizardman's people if they need help. Maybe they have a Shaman that has a nasty "potion" it knows how to make that enables its people to be under water longer...and if the party can stomach it, they can use it as well to have water breathing ability for an adventure against a common enemy. You can easily use a modified version of this for Mermen or Tritons as well (maybe the party does trade on a beach somewhere with a representative of either race...think ahead and have them make contact peacefully with a Merchant from one of these races. A Wizard sends them to buy pearls maybe?).
Nixies: I LOVE these little rascals! It takes 10 of them to cast a powerful Charm spell, but once they have you, they will keep you for a while! If you want your party to adventure in a large body of Fresh Water (only fresh water mind you!) then Nixies are a go to. They can peacefully approach a party because they need help against some underwater monsters...you don't even have to Charm the characters, they can be offered treasure by the Nixies (who can go onto land!) for helping them against something. What is more, each Nixie can cast a spell that enables a character to breathe water for "1 day" (so if your world has 36 hour days...great!). The Nixies can even give the party a base of operations like the old lair of a Giant Water Spider!
There are other ways to get the party under the water: Druids, Nymphs (as long as they stay clothed!), Sylphs and many others. You can essentially come up with just about any reason you can think of: The pretty Sylph approaches the party "I was flying over the lake and dropped my locket in it!" She then offers to use her magic to help the party breathe under water. Look at any creature with spells enough and you can find a way to get a party into even a long lost dungeon that is under water now because those pesky damn dwarfs built a dam and drowned the place a long time ago!
WEIGHT: Yeah, this one is easy! If you are worried your character(s) will sink like rocks...who cares! They can already breathe under water, so no problem right! This really makes things easier on the party honestly. If the party sinks to the bottom easily enough, you are essentially back to adventuring on land and worrying about aerial (don't ask me why they spell it that way!) attacks...except now pretty much everything flies except for the party. That is just pure win for the DM anyway right? Oh, you aren't a sadist like me when DMing? Hmmm...
Well, there you have it...just a quick way to take your game into another direction. I like to use Under Water Adventures even right next to a major city...who knew that long ago the river jumped its banks and drowned a dungeon! Well, that map the party has says the ancient wizard's tomb was right in the area...maybe THAT is what those crazy Nixies were going on about!
Trust me, once you get your party going under water, they won't be wet behind the ears anymore! Wait, that isn't right...