Hi Robert. The reason you don't hear much about it is that underwater volcanic activity does not heat the surface waters of the global ocean. We know this through several lines of evidence, the simplest being that the ocean gets colder the deeper it gets. If underwater eruptions or volcanic vents provided significant heat to the ocean, bottom waters would be hotter than the surface, but they aren't, as any diver will tell you.

And if increases in underwater volcanic activity were causing global warming, then we would expect that the deepest waters would be warming fastest and the rate of warming would get weaker the farther you get from the ocean bottom. Instead, it's the opposite: the rate of warming is fastest as the surface and slower the deeper you go. This proves that the cause of the warming is something at the surface, not at the bottom.

We haven't updated this graphic since 2019, but it clearly shows you waht I am talking about:

https://www.climate.gov/media/13056

The colored lines show the amount of heat build up at different depths. The most heating is at the surface; the least is in the abyss.