The type of metal detector you choose will depend mainly on what type of loot you are searching for and where you will be looking for it. There are generally three types of metal detectors for different purposes—land-based coin and relic hunting, beach and underwater detecting, and prospecting for gold.
Beach and underwater detectors tend to be more robust and rust resistant to help fight corrosion from sea air and salt water. Obviously, they need to be waterproof to detect through water and sand, which have different properties than dry dirt.
Gold detectors will use higher frequencies to detect the differences between gold and other metals. These will be most successful in gold-rich areas in the U.S. (generally the west coast), since you are unlikely to find any natural gold elsewhere.
A multi-purpose metal detector is best for beginners. It can find coins and other metal objects and can sometimes even sniff out some gold.