Saltwater has minerals in it that can affect how a metal detector functions but mostly, this refers to the VLF detectors as they are much more affected, heavily so, by the presence of saltwater and minerals. Using your metal detector in saltwater vs in freshwater is two different things. These will still work but just not as well as the PI models which are much more skilled at dealing with these types of conditions.
Saltwater Vs Freshwater
If you are only ever going to be diving into freshwater, then the VLF type of detectors would probably be better and they are less prone to detecting random junk and so, would be more accurate. If you are looking for an all-around model in all conditions, though, then it is best to go for a VLF type of metal detector.
Weight and Buoyancy
If you are using anything in the water, then you generally would want it to have neutral buoyancy so that it doe not sink too quickly into the water but you also do not want it to constantly try to float to the top, either, since that would be very difficult to control. It is a balance that is hard to find but one that is important when it comes to underwater metal detectors so finding the right weight is quite vital.
If you want to use the metal detector out of the water, then you also want to make sure that it is not going to be too heavy for you to use. All products will display their net weight, so, if this is a key consideration to you, then you should make sure that you are checking the weights and comparing them.
When you are using a metal detector over land, then you probably want the device to be as light as can be so that it will be easy to handle and easy to move around. In the water, though, if you were trying to dive with something that is light and very buoyant, then you would have a problem since you would be fighting constantly to keep it with you as the device will keep trying to float to the surface.