I once read a theory that this was what was taking place in the Bermuda Triangle.
As I heard it, a Ship sailing over an area like that would lose Buoyancy and sink.
That was what came to my mind as well.
In the Bermuda Triangle story, they talked about how maybe CO2 gas bubbled up from the seafloor. That since a ship floats on the basis of the weight of the water it displaces being more than the weight of the ship, if the water weights a lot less, because it is full of gas bubbles, the ship will sink like a rock.
I would think that the gas bubbles from a volcano would also be a danger to ships in the area, especially small fishing ships.
While talking a lot about gas bubbles, the story does not even indicate the risk to ships.