Here's why:

* Divergent Boundaries: At these boundaries, tectonic plates move apart. This creates space for magma to rise from the mantle, forming new oceanic crust. This process is called seafloor spreading. The majority of underwater volcanoes are associated with this spreading activity.

* Other Plate Boundaries: While divergent boundaries are the primary location, underwater volcanoes can also occur at:

* Convergent Boundaries: Where plates collide, one plate can subduct beneath the other. This process can also lead to volcanic activity, but it's less common than at divergent boundaries.

* Hot Spots: These are areas where magma plumes rise from deep within the Earth's mantle. They can occur in the middle of tectonic plates, creating chains of volcanic islands.

Let me know if you'd like to delve deeper into any of these types of plate boundaries!