Cutthroats (1984), a deep sea adventure by Michael Berlyn.

As described on the box:

You're about to get yourself into very deep trouble.

You're a backwater island's top diver and foremost expert on local shipwrecks. Which makes you perfect for the job a band of the island's shadiest characters has in mind for you. It's a simple business proposition: all you have to do is salvage a fortune in sunken treasures. You stand to gain millions. The only drawback is, it could cost you your neck. Because to successfully recover the treasure, you'll have to survive the perils of diving in unknown waters - and the even greater danger of an untrustworthy crew. But none of that will stop you from taking the plunge. You're the type who believes that when stakes are this high, even when your odds are this low, it's worth running the risks of dealing with CUTTHROATS.