From what I know water provinces are not affected by temperature scales. Nor cold nor heat. You take cold just because it is usually more useful [water mages].
Cold benefits underwater creatures by increasing the oxygen dissolved in it for them to breathe. Heating it up allows the oxygen to escape to the air above and then the fish suffocate, the cause of many fish kills in a hot summer.
Implementing this within the present game mechanics is tricky. Underwater creatures should definitely get the "cold blooded" tag and be injured by cold if they come ashore. But while underwater, the affect of the cold on their bodies is probably balanced by the increased O2. So no penalty or bonus for cold, but a penalty for heat underwater makes some sense.
The Visitor is hampered by moving through water, it's annoying. And yes, it CAN lead Vastnesses, it gets ~160 Magic Leadership. This makes them the ultimate overland raiders IMO, even though to fully exploit that mobility, you're either attacking blind a lot, or you have some pretty intense scout saturation.....