We don't like killing the wildlife and if the bodies aren't noticed this can quickly foul the water and make it unsafe for your horse to drink. Fresh water is essential to your horse's health.
We remedied the problem by leaning a small clean log on the side of the trough so it extends into the water. This allows any critters that fall in an escape route. Be sure to use a piece of non-toxic and untreated wood.
Keep your horse water trough clean by scrubbing it out occasionally, especially if algae tends to build up in warm weather. If there is a drowning give the trough a scrub with bleach or vinegar, rinse very well, and let it dry thoroughly before refilling.