Based on your description, it could be a sponge (but I don't think sponges have a contractile skin), a tunicate or a sea cucumber.
I would favour the last option.
- The following questions might help further identification: How many holes can you spot?
- If there are 2, where are they placed with respect to one another?
I hope someone can propose something more precise.
As noted in the comments, it could also be a sea anemone. In my experience, these have a quite soft body. In that case, it should have only one opening, and the opening should let tentacles out when the animal is left quiet in the proper environment.