I don't think Marty and his partner Craig Tester would have gotten involved in Oak Island at all on their own, for the same reasons that have been pointed out by many. There's no evidence of a treasure that might have ended up on Oak Island. The Knight's Templar story sounds like it comes out of a Dan Brown novel. I haven't seen any solid evidence that any such treasure even got out of France, let alone to Scotland and then to Oak Island. And the only thing we have evidence for on Oak Island itself is the existence of treasure hunters.

Marty and Craig did in fact invest in treasure hunting in the Caribbean/Florida area. This was revealed in Beyond Oak Island. There, you've got actual treasure to hunt for: sunk Spanish galleons carrying gold back to Spain. You've got several such finds already on the books. And they invested in a very experienced group that already had finds on their resume. But they ran into big legal troubles with the state of Florida over the rights to excavate and recover potential treasure from sites they wanted to excavate. Apparently, their investment there came to naught.