Water Eggman (ウォーターエッグマン), also known as Egg Poison, is the second boss of Sonic the Hedgehog 2, appearing at the end of Chemical Plant Zone.
This is a tricky boss in that the floor panels on each side of the boss arena flip over, kind of like trap doors. The player has to be careful not to be standing on a platform when it turns over, or they'll plummet to their death. The trick here is to simply bounce on Dr. Eggman twice in a row while he's docked on the side of the screen, filling up his giant vat with blue chemicals. When that's full, he'll try to dump it on the player, though this can be avoided by jumping over the chemicals, which is not hard to do. Alternatively, if playing as Sonic, the player can duck when Eggman drops the chemicals; it won't harm them. After eight hits, the boss is defeated.
It is possible to defeat Eggman in one go if the player stays on top of him when he enters the arena. Unfortunately, due to timing issues, this will lead the player to fall to the platform while the surface is underneath, causing death. The remedy for this is to count to seven hits, then pull left to land on the middle (static) platform and dodge the chemical drop to hit him once on the left hand side. In the 2013 remake, this is the easiest way to get the "Bop Non-stop" achievement.
In the final build and all prototype builds of Sonic 2, sprites have been found of Eggman accidentally dumping the chemicals in his machine on himself. This may be a missing boss ending sequence similar to the one in Emerald Hill Zone.
Retro engine version
The player cannot duck to dodge the drops in the 2013 remake and the Sonic Origins version.