underwater combat and movement

[Lance Tankmen]

So i don't often have to deal with water and the like but i did a while back and luckily it was only about 10-20ft deep kind of thing but are there rules anywhere on if wearing heavy or medium armor affects a character or if weight matters for swimming that i may of over looked? i find it impractical for someone in plate to be able to actually swim but if its not a set rule already id need to think of some minor fix.

[Chugger]

Book not on my, but I'm pretty sure there's a section on underwater combat in the PHB (possibly more info in DMG).

IIRC fire spells don't work so well - you get resistance if mostly submerged in water if hit by a fire spell. If you don't have a swim speed or magic underwater movement, you're limited in movement and even what kind of weapons you can use well (tridents work - stabby items work - all weapons work if you have magic underwater movement or perhaps racial by being that race or doing some sort of spell). Just check the indexes.