I have done this many times and i just wanted to post my info as to possibly aleviate headaches in the testing end of it. The easiest way to replicate this is to mount up and jump into deep enough water for your horse to auto-dismount. I know stupid post as it has already been posted similarly, however it might prove usefull in testing. It may just be as simple as scripting the auto-dismount in the air after a jump. As far as I know as of yesterday if I was mounted and jumped, hit deep water with my horse ..sprinting or not... it auto-dismounts and you swim at the current speed. Hope this isnt the dumbest post you ever seen.