Depending on what you looking to dive there are many options. In the St. Lawrence area Hunt’s is a definite dive shop in the area on the US side. T.I.D.E - Thousand Islands Dive Excursion is a charter on the US side. He is very experienced and dive several wrecks like the Maggy L, Vickory, Keystorm and America.
There are also several dive shops on the Canadian side offering shore dives and charters.
If you head to Alexandria Bay there is a public area behind the hospital. It’s a shore dive on the Wreck of the Islander. There is also a table and chairs set up at about 55’ many divers go to. l