After Wallace kills the noble in his bed with the ball-and-chain, he jumps his horse out of the house into the lake. If you watch the horse as he falls, you'll notice he doesnt move at all. In fact, he's completely stiff and lifeless (absolutely no animals were harmed in the making of this slip-up message)
Actually this was very good attention to detail, because a horse cannot land in the same way a human can and the jump would in fact have killed the horse, which is why it floats there, dead.
Yes, the horse looks very fake right before it hits the water. Horses do dive into water. They could have used a trained diving horse, but maybe they couldn't find one that looked like the black horse.
But you have to admit, that was a really well trained horse, doing all that stuff going into the bedroom, stepping up on bed, and rearing against doors.
But yes, the horse was very fake looking. And if the horse did die, I seriously doubt it would float around like that. But, this is a MOVIE. I thought is was great, even if they did use a fake horse :P