First of all I know nothing about md'ing underwater. Never even you could be submerged in the water with a md. So I have a slight interest in trying it out. I see a lot of detectors that claim units can be submerged up to 100+ feet. So lets say I wanted to scuba about 6-14ft depth in the Chesapeake bay on average (I live maryland). What are a few units that would work in these conditions (mild-medium salt water). I see some on kellyco for like $300 but im a bit skeptical. Best option for the best $$?
Leopard11, as rook3434 says, mineralization is a factor to take into effect. But in my opinion, that's rarely a factor in any place you can scuba too. No more so than above water beaches: there are very few that restrict anyone to *just* a pulse unit. I've only seen a few that back, and even then, only in select stretches (water/channel culvert washout, etc...). The vast majority of beaches (and underwater) will allow for standard detectors. Sure, if it's starts to get nasty, you have to swing slower, and would loose depth (in gun-powder grey colored minerals), but ... there's perpetual tradeoffs:
Because a pulse unit won't have the ability to eliminate iron. Yes it's easy to use, and yes it effortlessly sees through nasty minerals to find the daintiest of chains. But if your site is riddled with iron, you will be hating-life-indeed.
There's the cz20 or cz21, which are easy to use. But they are silent search (no threshold), which some hunters don't like. And you will find yourself "holding your breath" (d/t the noisy bubbles around the facemask) to hear the fast "beeps". Hence I didn't care for in scuba. There's the excaliber which solves that problem, by the tell-tale long beeeeeeppp (with a tail). You can't miss it, even in the underwater noisy world of bubbles and such. But it takes some getting used to. The "flock of sick geese" tooty-fluty sounds would require you to practice ad-nauseum above water first, to be familiar with the symphonic sounds.
So there's pros' and con's, depending on what you're used to, what you're searching for, what your junk tolerance (and environment) is, etc...