Where are you going to be using your rc boat? A stream, pond, lake perhaps? Or maybe a pool. In order to decide on which type of radio controlled boat to buy, you need to know what is allowed where you intend on driving your boat. Are there any noise regulations to consider? If so, you may want to invest in an electric powered boat verses a fuel powered one. Fuel powered (gasoline or nitro) boats tend to be louder and may not be allowed in your specific area. If you have limited water space such as a pool, you may want to consider a mini rc boat.
Do you want a boat that is ready to run or a kit that requires assembly? Ready to run kits are more expensive than assembly required, but if you are just getting into radio controlled boats, a ready to run kit that requires no assembly and comes with all the controls may be the easiest way to enter the hobby.
Consider your level of experience. Most starter kits usually come with drive systems with submerged propellers. These systems cause more drag and therefore less speeds, but are easy to run and maintain.