Generally, underwater machines are category specific and would not be good for land hunting. Beach and underwater machines are designed to deal with the high mineralization of the sandy beach area. Gold nugget machines are also in a class of their own. They re designed for hunting small flecks of gold. And there are relic machines made primarily for hunting buttons, buckles, bullets, etc. on historical battle sites.
Most beginners consider a coinshooting or multi-purpose machine.
Be wary of very low end machines like the ones for under a hundred dollars. Sure they ll find metal but having the ability to discriminate between a piece of scrap iron and say a dime is a crucial issue. No one wants to spend all day digging bottlecaps, nails and junk. Top end detectors run well over a grand. And that s just for the machine.