The mob that cannot attend pool parties because water makes it flee toward its “block form” is the Enderman. In Minecraft, Endermen take damage when they touch water, rain, or other water-based effects, so they rapidly teleport away to find a dry location.
Endermen are also famous for their ability to pick up and place certain blocks. That unusual behavior explains the joke’s “block form” wording, although an Enderman does not literally transform into a block. Its real water weakness is a core gameplay rule: contact with water causes damage and repeated teleportation, while rain can make it blink around continuously until it reaches shelter.
The other choices do not fit. Creepers are known for exploding near players. Zombies and skeletons are vulnerable to sunlight, but water does not make them teleport away; in fact, both can survive underwater, with zombies able to become drowned. Endermen were introduced in Minecraft Beta 1.8 in 2011 and remain one of the game’s most distinctive hostile mobs because they combine teleportation, block manipulation, and a dangerous reaction to eye contact.