There 3 types of water shoes mesh tops, neoprene tops, and sandals. All 3 types share a couple of features such as 0 bouyancy and have a tough but very flexible bottom to prevent feet from being cut up on rocks, shells and coral, as well as protecting them stingers, spines, and teeth of marine life.
Inexpensive mesh top ones can be had at places such as Dollar General and 5 Below. The 3 bad things with these is they generally come in split sizes (ex 9/10 or S, M, or Lg) providing imperfect fits, the mesh lets sand & other very small particles in easily but not out again, and generally only last 4-5 wearings before the glue gives out or the bottoms crack.
The neoprene ones are perhaps the most popular and prices range widely on these depending on where you buy and the name brand (or non-brand) you decide on. No names as well as last year's styles of name brands can be had at places like Target and Wallyworld for under $20 in the seasonal section spring to early summer. While new style brands names (Speedo, Body Glove, and Under Armour) can be found year round sporting and marine equipment stores like Modells and West Marine starting at about $30 and going up to over $100. The advantage to this type is when fitted properly and the ankle cinch pulled not much gets into the shoe.
The sandal type is more popular with people that like to go from land sports to water activities without stopping to change. These tend to be the most rugged, being able to handle trails even rock climbing then water sports even white water rafting. If this style is your choice, avoid no names brands for these and stick with Trekkers, Teva, Lands End, and LL Bean. These generally run $70 + & can be found at sporting equipment stores, but if confident on the size you need you can save money ordering direct from the vendor or from outlets online.