OK, maybe not swim, but when I saw them in Spongebob scooting around, I thought it was just to give them a butterfly equivalence.
I thought they just sat on the ocean floor like an oyster, barely moving via muscle foot.
They are filter feeders and use a siphon. The siphon can expel water under pressure and move them.
I’ve wondered why there are no creatures that use peristalsis as a form of locomotion through water. (Earthworms use it to propel themselves through soil.) If a creature had a tube that ran the length of its body, it could conceivably use muscle contraction in waves to create a continuous jet of water for locomotion.
Probably energy efficiency. Bursts take less energy and give the muscles a short chance to rest vs. continuous movement. In water I would think the peristaltic muscles would not get to rest because they’d be in a constant state of flux to maintain their forward momentum (since water currents could continually push them back - something Earthworms don’t need to worry about with soil).
Makes sense, but it would be a good sci-fi creature. In modern vertebrates, it could just be a modified digestive tract, which already uses peristalsis to move food along and digestive cilia could presumably be adapted to filter feed.
Sea cucumbers are basically ambulatory tubes, very simple circulatory system (no blood, only seawater), invertebrate, just a mouth at one end and an anus on the other. But they walk around on little feet like starfish, no jet propulsion.