OK, so you want to look for sunken treasure – be it monetary, historical, or both. (Or you find out someone else is doing it, you don’t think they should, and you want to see if you can stop them).
You’re in luck, sort of.
The bad news for underwater artifact-hunters is that unless the body of water is completely surrounded by private land, you’ll probably have to get permits from, comply with the regulations of, and possibly split the loot with, some government or other. Even if it is on private land, some environmental laws may still restrict what you can do. The good news (for both hunters and their opponents) is that now it’s well settled which governments have jurisdiction where. This issue was hotly contested between the coastal states and the federal government in the two-decade U.S. v. Florida series of cases.
I’ve prepared a diagram* that shows the types and extents of underwater government jurisdiction in the United States. Click the thumbnail image below to enlarge it: