Well, of course, all arctic and antarctic animals are acclimated for their biomes. Some with a kind of antifreeze in their blood, and various other adaptations from their more temperate-climate oriented counterparts.
Although they are LAND animals, Polar bears spend MOST of their time in water, even in open ocean, iceberg to iceberg, ice floe to ice floe—eating fish, but often times are ravenously hungry when they reach land.
And by the way, they DO live on land. Polar Ice melting is more of a threat to global ocean levels than to polar bears’ living space, because that would be the land. Ocean levels could rise and make less land, but melting ice doesn’t mean they’ll drown.
So, no deaths or incidents form the plunge. They are adapted for it.