This article is from the Final-Fantasy and other Square Soft Games FAQ, by firstname.lastname@example.org (Nick Zitzmann) with numerous contributions by others.
To kill the Waterpolos, cast attack magic on them, or use a Summon
spell. Your character will take no damage, but the Waterpolos will die
if you did enough damage (they have 40 HP). Any Waterpolos that are
on-screen when Bottom Swell is killed will also vanish. When a character
is encased in a Waterpolo, they lose life at a rapid pace, so act
quickly before your ally is killed.