Possibly the best salt water detector is the Excalibur, if for no other reason but that it seems to be the most popular. The Garrett Infinium is a contender, as it is a PI and the only one with features that help ID iron. I personally like the Fisher CZ-21dual frequency VLF, but then I like Fishers in general. I would rate the Whites models at the back, but serviceable. I've seen "issues" with them, so I lack confidence in them, compared to these others.
None of these are greatly suited for the land, however, when properly set up for the salt beach. The Infinium can interchange coils, so it works on land after a fashion. But really, you should not ask one detector to work well in both the salt and land environments. What works well in one, means compromises and perhaps poor performance in the other. Most water detectors are needlessly heavy, even clunky, for use on the land, as well. Heck, the Excalibur even needs a support harness to be it's best down at the beach, the environment it was designed for! Can you imagine how it would be at the local park?
If you are going to be a dedicated water hunter, then have a dedicated water unit.
The overiding concern for any beach detector is IRON. You must dig the other targets all the way down into the foil range, if you want the gold, so you really only want iron ID of some sort. PI's don't have it as a rule, although again, the Infinium offers some ferrous audio indication - not foolproof, but better than nothing. Without iron ID, you will end up moving mountains of sand for rusty fishhooks, bottle caps and nails. All of the VLF models worth considering are good iron ID'ers.
The other concern is depth. Sand shifts at the beach, and what was shallow yesterday can be deep tomorrow. PI's give the ultimate in depth, and so are a wise choice for the ocean beach. Here again, you are moving lots of sand, with no assurance that you are not digging up an encrusted iron "blob". The mulitfrequency VLF detectors are a step in the right direction, here with their better iron ID, and give surprisingly good depth themselves.
SO where does this leave you? If you are on a budget, I would consider a used Garrett Sea Hunter II, CZ 20 or Whites multifrequency VLF units (this is one of the only times I'd suggest the Whites - when buying used as a first timer).
If cost is no object, I would spring for a new Infinium or CZ-21. Both are top of the line, rugged instruments that work well in either fresh or salt water.
