Flippers wouldn't help really given how horses actually swim, they kind of kick back with each of they're legs in a sequence similar to a trot. Their hooves already have a bit of a hollow which likely helps with swimming. Honestly though horses are pretty good swimmers, because they have huge lungs which helps them stay afloat with ease
Just wanted to add to this for those who don't know, windows games work through a comparability later called Proton, it usually works great, but some games don't work well with it. (Mostly anticheat and stuff like that causing issues IIRC) I would always recommend checking ProtonDB before purchasing any game without explicit Linux support