Acrylic float is big and hollow inside, if they’re really solid. If they’re big Piece and they’re floating, they’re probably hollow.
If they’re little piece, then the problem is surface tension. Acrylic is not very well wetted (that is, the water doesn’t like to touch it), so small pieces are light enough that they won’t push their way into the water.
If you still want to use small pieces, look for little pellets or beads.