Swim shoes or water shoes primary purpose is to protect the the feet from sharp objects, slippery surfaces such as the deck around a pool and from stepping on small rocks.
The most important thing to look for in a good pair of water shoes is a tough, durable sole that will protect your feet. Try a product with soles made of firm material that also has a good tread that will allow your feet to maintain a solid grip on slippery surfaces.
Swim shoes should also provide full foot coverage. Some water shoes have zippers. Try a snug fit so your foot will not slip inside of the shoe.