Significant damage. At least enough damage that the ship would be out of action for a significant amount of time. A whole lot depends upon what munition a diver attached and where specifically the munition was attached.
Stay well - Stay safe - Stay armed - Yorktown
There's a guy who's been warning of this sort of thing for years- since he could surely do it with some crews and enough money to get them where they need to be. I think he said he could sew up he fleet for at least 18 weeks with only the most rudimentary of explosives, and seemed more concerned about dolphins than other more conventional countermeasures, many of which weren't even being used because training is down so bad and people have gotten lazy over the past years. But no one paid much attention... everyone with the rank needed to get anything done was more concerned with maintaining the status quo until they could retire than risk getting involved in any serious effort to fix the weakness. And no, we don't have enough dolphins.