Why can shallow underwater volcanic cones end up with flat tops at nearly the same depth?

Answer: Storm waves reach only so deep

Lava spreads evenly as it cools — Lava can spread, but it would not naturally trim many separate cones to the same water depth. Flow shape depends on eruption volume, slope, and cooling, so the tops would vary more from cone to cone. A shared shallow cutoff points to erosion controlled by how deep strong waves can stir the water.

Storm waves reach only so deep ✓ — Storm waves have a lower reach, often called wave base. If a volcanic cone or short-lived island rises into that zone, waves can plane off loose volcanic material until the summit sits below their strongest motion. That makes the flat top a cut surface, not a surface the volcano built flat on purpose.

Ice pushes the summits down — Ice can make flat-topped volcanoes on land, so it is a tempting Iceland-related guess. For these shallow seafloor cones, the better clue is that storm waves repeatedly attack roughly the same depth range. Pressing from ice would not explain why separate submerged tops cluster near a wave-controlled cutoff.