We bought one for my step daughter (she's 25) for Christmas.
It IS a Prism III with a couple physical changes: The Coimaster shares all of the features of the Prizm III without a locking collar on the upper rod, no arm strap and an un-printed box. Basicaly it's a prizm III. Quiet no fuss detector @ 8kHz.
The prizm III like all single frequency detectors will be affected by salt water to some degree.
To alleviate this effect you can lower sesitivity levels and/or increase discrimination slightly.
Small coils will be less affected by ground mineralization in general and would probably help, but
the beach is a big area to cover with a shooter coil..
It does fine in and around the fresh water, and has a silent search.
Compatable coils would be all Prizm coils to include: 950, 9" Spider, 8" Flat bottom, 4x6 Shooter DD.
As for results, we tried it out one day, then the snow flew.
My stepdaughter found wheaties and other coins her first time out in her front yard. We figured the yard would have some trash, but the Coinmaster pulled the coins into signal from the turn on and go settings. She even pulled a coin in the same hole with some trash and the detector never sounded off on the trash (that can be changed with discrimination). The point is, the turn on and go settings are nothing to mess with until you get a feel for the detector.
In my opinion, it's well worth the money. You get a $250-$300 machine for $180. Well worth it. esale value may not be great, but with a machine like this, you'll want to keep it at least for a backup machine.