The oceans are also pretty cold as you go down, since the cavern-deep surface oceans will probably be a seperate climate entirely diving or being dropped down there could be uncomfortable. Those crab-people would either need a limited range, clothes or/and thermal competent tissue layers.
Even in tropical waters on the surface, a human will die of hypothermia after a few hours in the ocean. Water is much more efficient at absorbing heat than air is, and the ocean is an effectively infinite sink. In very cold waters, the only warm-blooded animals that can survive are all notably huge creatures (polar bears are the largest bears, pengiuns are very fat, and then there's whales...), while everything else is cold-blooded. Cold-blooded creatures can even live in lakes that are frozen over at nearly all times, just because if they operate at extremely low temperature and speed, they can stay very energy/food-efficient. Meanwhile, blue whales need to ram-intake literal tons of krill every day to stay alive.
That said, oceans tend to be banded into temperature ranges that are fairly broad. The thermal layer can move up and down, and the difference in layer water (including salinity as well as temperature - remember, hot water rises, but the sun warms the top of the water, not the bottom, so there's no reason for the upper water to mix with lower water like hot air near the surface of the earth convects) changes when it rains, but a fairly broad band below the layer would be roughly similar, provided you're already adapted to low or no light. The ocean floor is also pretty huge stretches of "wide open plains" as it were. There are shelfs here and there, but if you were somehow adapted to a specific depth, the ocean floor stays roughly the same depth (give or take 100 feet) for continent-sized areas of the planet.