All good advise above.
My two cents on the underwater units: You will find more stuff at the beach in the water (vs the dry), so a detector that can be used to at least wading depth is a plus. For both nugget hunting and beach hunting, if you get a land-only detector, then it is helpful if at least the coil can get wet. The big issue on underwater units is they are heavy ... you can use them in the dry, but if you are detecting for a while, they will fatigue your arm.
My best advise is to identify the kind of detecting that you will do most and get the best detector for the price (used or new) and your location. Use it in your secondary kind of detecting, but at some point you will probably buy a different detector for that. This way, you have not compromised too much on your initial detector.