... gas trapped inside the black volcanic material.
Sudden release of such gases has been suspected in the case of a number of sudden ship disappearances over the centuries. If a bubble's volume is large relative to the ship's, the ship will drop to the bottom as the gas forces water from under the hull. Then the surrounding seas crash onto the decks.
Try to picture this. All the water next to a ship mysteriously flowing away from the ship, over the bubble while the ship for some reason is the only thing going down through it.
Water is also heavier than air, and water on top of a bubble will also fall into the center of the bubble (breaking it up into smaller bubbles), until the surface tension of the water around the bubble is great enough to hold it together (small bubble).
Therefore a huge bubble gets split into many bubbles, I suppose at depth a lot of the gas gets dissolved into the water. The worst that would happen is many small bubbles cause the density of the water to decrease, causing the boat to sink lower into the water. I'm not sure if the water could lose enough density to sink a regular boat directly?
I think what you may be thinking of is large, shallow explosions (generally man made) under a ship which create a temporary bubble which then collapses in on itself, but can cause extreme stress on large ships.
There is a no-sail zone quite near the coast of Grenada due to an active underwater volcano. The volcano is called Kick 'em Jenny and has sunk numerous boats over the years. On one instance it sunk a boat and killed 60 people.
Wikipedia link for the lazy https://en.wikipedia.org/wiki/Kick_%27em_Jenny#Maritime_excl... The part about the "sinking hazard" has a [citation needed] anyway. The Spanish version look more concerned about eruptions than bubbles, and says that when they detect unusual activity the radius is increased from 1 mile to 3 miles.
Bubbles that large are not stable—the sinking effect (which is disputed) would have to do with a significant reduction in density of the water by a large number of smaller bubbles.
Sudden release of such gases has been suspected in the case of a number of sudden ship disappearances over the centuries. If a bubble's volume is large relative to the ship's, the ship will drop to the bottom as the gas forces water from under the hull. Then the surrounding seas crash onto the decks.