Eyjafjallajökull was a special case, it was not an oceanic blast, but a blast under a glacier. Fjallajökull means "glacier mountain". The blast flash-melted several tones of ice, which caused the lava to cool rapidly forming the ash cloud. If my memory serves, the glacier was in the caldera itself.

But the effect would be very similar. The water would vaporize and create an aerosol of ash.