Minelab X-Terra Pro, fully waterproof and it has an actual saltwater beach mode
Minelab Vanquish 440 which has the minimum amount of features, the 340 is a great little detector but no pinpoint mode and no iron audio On/Off among other things, the 440 also has outstanding simultaneous multi frequency beach capabilities but only the coil is waterproof. Really accurate target ID system
Nokta Double Score, simultaneous multi frequency, fully waterproof and lots of good features,
Nokta Simplex models, fully waterproof but you had better really like the tones on the Simplex detectors. I would not recommend buying one unless you have heard what they sound like compared to the ones listed above.
Nokta Legend, awesome full featured, fully waterproof detector and there is a used one for sale in this forum's classifieds for $330 which is a total steal. You won't need to upgrade for a long time.