Big Shaker (ビッグシェイカー), the sub-boss at the end of Hydrocity Zone Act 1, is a spherical contraption with four rockets. This device spins back and forth through the u-shaped pit in which you are standing, before settling on a pole set in the middle of the arena. It's only safe to attack the device while it is in the middle, at which time it will have begun to turn the pole, which in turn stirs the water, and your character. However, you can Spin Dash up the side and attack while it is off-screen without getting hurt. Though the boss itself is relatively simple to beat, remember that Sonic and co. can still drown while underwater.
This mini-boss is strangely the only mini-boss in Sonic 3 to use the full boss theme normally reserved for Act 2 bosses. This is changed in Sonic 3 & Knuckles, in which the normal mid-boss theme for that game is heard during this boss fight. Also, if you stay in the water long enough for the drowning music to start, when you come out, the miniboss music from Sonic & Knuckles plays instead of the Sonic 3 miniboss music.