Sea cucumbers are one of the safest creatures under the water to touch.

Sea cucumbers are echinoderms—like starfish and sea urchins. There are some
1,250 known species, and many of these animals are indeed shaped like
soft-bodied cucumbers. All sea cucumbers are ocean dwellers, though some inhabit
the shallows and others live in the deep ocean. They live on or near the ocean
floor—sometimes partially buried beneath it.
Sea cucumbers feed on tiny particles like algae, minute aquatic animals, or
waste materials, which they gather in with 8 to 30 tube feet that look like
tentacles surrounding their mouths. The animals break down these particles into
even smaller pieces, which become fodder for bacteria, and thus recycle them
back into the ocean ecosystem.
When in danger, these animals will expel their viscera together with the
Cuvierian organ. In the water it becomes larger and splits into long white
sticky threads that adhere to the enemy’s body. The venom of the sea cucumbers
quickly weakens the muscles of the enemy and if the Cuvierian tubules come into
contact with the eyes, the result may be permanent blindness.