Maxperson
Morkus from Orkus
Even if doing that was in the rules(and I'd allow it anyway if a player did that), it still wouldn't matter much unless the PC fell into rapids or something. If he fell into calm and even slowly moving water, the swim DC is going to be very easy, so a DC 5. Since I already specified he has athletics and is good at climbing, say an 16 strength, he's has +5 to the roll at 1st level. Even with disadvantage he will always succeed easily at that swim since 1's don't auto fail for skill checks.Sure, but if swimming came up you could just refuse to have the character try it. Or if it was unavoidable (like if the PC somehow fell into the water) you could request to roll with disadvantage because of the flaw. Something like that.
I'd love to see a skill system in-between 3e and 5e. Less grouping, but not a zillion different skills.