Water Eggman

Water Robotnik (ウォーターエッグマン) is the second boss of Sonic the Hedgehog 2, which the player fights at Chemical Plant Zone Act 2. To defeat Water Eggman, the player has to hit eight times to the Eggmobile to defeat it.

Water Eggman is mostly an Eggmobile with a tube-like crane that releases Mega Mack. It is mostly grey, and has black and yellow stripes. It is piloted by Doctor Robotnik, who after filling the canister fully, tries to drop the Mega Mack to the player's neck.

Battle
The boss battle takes place on a yellow platform over a lake of toxic purple liquid (Mega Mack). On either side of the arena, there are three platforms that flip over and drop anyone standing on them into the Mega Mack, which spells instant death. The player has to time his jump to the vehicle for avoiding to fall into Mega Mack ocean.

The boss itself is Dr. Robotnik in his Eggmobile, which has been modified with a canister over Robotnik's head and a system of tubes that can fill the canister with Mega Mack from the lake beneath (although the liquid in the canister appears blue rather than purple). When the canister is full, Robotnik will attempt to dump the contents on top of the player. After that, Robotnik moves to the other corner to fill another amount of Mega Mack to the canister and repeats the same pattern.

The player either can jump, move away or crouch, as the dropped Mega Mack won't deal any damage. The Water Eggman takes eight hits to defeat, after which the player can open up the capsule and end the zone. In locked-on version of Sonic the Hedgehog 2, Knuckles may have to time his lower Spin Jump into Robotnik to avoid falling to the Mega Mack ocean.

Trivia

 * This boss can actually be defeated very easily by running right up to him at the start of the battle and bouncing off him repeatedly while against the right edge of the screen. No controller input is even required at this point.
 * In a prototype version of Sonic the Hedgehog 2, sprites of Robotnik have been found where he took the chemicals and accidentally dumps them on himself. This might have been an ending sequence for this zone, but was removed.
 * It is one of bosses that don't have Star Post located near of the battle arena. Between two the closest Star Posts (upper and lower pathway) and the boss arena have slight distance to tread by.