Its hard to tell from just the photo and brief description, but the website above has a key to Caribbean sponges (Phylum Porifera), and that's my guess. Click on the button on the right. Keep in mind, it may be a decayed specimen of a deep-water species, washed up on the beach, so it may have little resemblance to the living animal, and it may not be familiar to beachcombers or snorkelers. Good luck.
It also might be a colonial ascidian, a form of tiny sea squirt that comes in clumps of many individuals. Does it have a holdfast or attachment? Is it grainy or gritty in internal structure, or does it have a multicellular appearance to it? Does it have internal channels and passages, or is it a solid mass of tissue. Does it have anything that looks like differentiated internal organs? Or is it a solid mass of tissue? Did it appear fresh, or decomposed? Some of these critters look very different when they start to rot, compared to the appearance when fresh or alive.
It reminds me a bit of the N. Pacific Gumboot Chiton, although that is a mollusc (even if it certainly doesn't look like one).https://en.wikipedia.org/wiki/Gumboot_chiton