.... machines designed for use in water are by their design "water proof" thus usually heavier, perhaps bulkier, perhaps less ergonomically balanced than machines designed for beach and inland use.
Water machines need weighted or neutral buoyancy coils to keep them from floating. You want as light a coil as possible on land. Water machines are heavier to keep their water proof integrity. Land machines are almost always lighter than water machines.
Land machines give you more / better target info like guesstimate of metal type and depth of target. Water machines typically give single tone or a few distinctive tones to ID the target and do not tell you the target's depth. Land machines start at the low end of price with single tone and offer full range tone ID and other user features as the price increases.
Land machines usually have water proof coils but the electronics package can not get wet thus should not be used in water more than a couple inches deep. Water machines can be used in the water and inland .... but the trade off is more weight, less target info, a more cumbersome machine to use on land than in the water.
A good water machine will start in the mid to upper $500 and go up from there. A good land machine will start at about $200 and go up from there. As in all things purchased, you get what you pay for.
My advice to you would be to decide what type of detecting will you be doing the most? If you are going to be spending less than 10% of time in the water get a land machine first. Save up and get a water machine later. If you live in an area where you can water detect almost every day of the year, like here in SW Fla, and you plan to spend well over 80 %of your time in the water and 20% on the beach and very little inland detecting, buy a water machine.
If you were to ever determine if you would enjoy boating, would you buy an inner-tube as your first watercraft? Save up your money then buy the best machine for the type of hunting you plan to do the most ... that current cash flow allows. Hope this info helps.