The Old Crow
Explorer
Did you miss the entire discussion of the desire to make water an actual consideration and possible hazard? If swimming is automatic in every situation, then that takes a great deal of interest and variety out of combat encounters.
According to RAW, swimming is automatic unless the DM indicates otherwise. I'm indicating otherwise because it makes combat on a boat, with the bad guys trying to throw you overboard, interesting.
Making wearing heavy armor more hazardous for those who wear heavy armor makes wearing heavy armor more hazardous for those who wear heavy armor. For water to be hazard, I would consider currents that push in directions the swimmer does not want to go, whirlpools that suck them down, hidden sharp rocks, difficult terrain like weeds, poor visibility like weeds or silt, icy temperature, creatures that grapple and pull adventures or innocents under, nets, paralysis, or the panicked traveller who actually doesn't know how to swim and needs to be rescued.