Heavy armor = glub, glub, glub. YMMV.
We had this debate before AIR. Basically IRL it is easy to swim forwards in metal armour, but
impossible not to sink, because the armour makes you much denser than the water. So crossing a small lake or river underwater is quite possible, but going in the ocean in metal armour is certain death.
For characters in magic armour I might be inclined to allow Athletics rolls to stay afloat briefly, but certainly if you put on non-magic plate armour and dive in the ocean, you are going to sink.