Surprisingly (or not) they are very good swimmers. One of the dressage routines at the Olympics consisted of the horses showing off their backstroke prowess. Unfortunately they stopped it after the drowning of 6 riders in the 1924 Summer Olympics in Paris (Gay Paree as it was known back then). The 6 riders were disqualified after failing to don the regulation sub aqua equipment (an 8ft length of rubber hose).