Brad,
Welcome to the forums and MD'g!
A Pulse Induction (PI) style detector is designed to ignore the effects of salt and moderate mineralization. Therefore you often achieve significant depth advantage over other single frequency/VLF detectors. The SS and Garrett Sea Hunter are two PI models which are very capable at ocean beaches as well as are waterproof so you can hunt in the surf or even dive with them. To buy either of them new, your cost is in the mid $600s from most retailers. Used, they run anywhere from $250-550. The only downside is that PI style detectors are not usually able to discriminate targets effectively, so hopefully you don't mind digging every signal.
Fisher has several CZ model detectors (CZ-7, 7a, 7a-pro, 70, 20, and 21) which are all great for the beach. They are dual frequency machines and able to better compensate for the ground conditions. The 20 and 21 are the newest models and are completely waterproof. New is out of your budget, but used ranges from $500-800. The other other models are splash proof (OK to get in the rain but not be submerged in water) and they also have a nice visual display screen that tells you the target ID and other settings. Used you can find these anywhere from $200-450. Also, these detectors have pretty good discrimination, unlike the PI. You can tune the detector to ignore targets like bobby pins, fish hooks, iron junk, bottle caps, etc.
-David