The Grubber is a hostile mob found in almost all varieties of ocean biomes. It does not spawn in deep warm oceans, as that biome was removed before grubbers were added. The grubber is a bottom-dwelling creature with little crab-like legs it uses to scutter around the seafloor. The grubber has an immense set of mandibles, or perhaps a single claw, atop its head, and can rocket upwards 8 blocks and clamp down on any mob, or player, that swims above it. The grubber can also pivot in any direction (aside from straight down, of course) to grab anything that gets too close on the seabed.
The grubber is an aggressive carnivore, and will gladly devour fish and players alike. It will not eat guardians or elder guardians due to their rocky, spiky bodies, nor will the grubber go after drowned, giant siphonophores, or skeleton fish, due to their lack of meat. Grubbers also seem to have a symbiotic relationship with the tide pool worms.
For any other mob that wanders too close, the grubber will grab them, dealing 12 damage in one hit on hard mode. The mob or player is held by the grubber using the same mechanics as a mine cart or boat, aside from the fact that the player cannot simply exit the same way they would from a vehicle; Instead, the mob/player is released if the grubber takes 10 damage or more. The grubber will damage the player/mob instantly upon first grabbing them, then every 2 seconds afterwards, until it's done at least 12 damage. After it's done at least 12 damage, the grubber will release the player after holding them another 2 seconds, regardless of the difficulty mode. (On hard mode, this means simply biting the player once, then releasing them 2 seconds later, but on other difficulty modes, they will hold on for longer unless detached by force)
Grubbers are aquatic mobs, and will die without water, but can survive on land for a full ten minutes. Grubbers are still hostile on land, but cannot do their high jump, instead becoming slow moving and easily avoided mobs that can easily be picked off.