It does float, though not necessarily at the top of the water column. It's also larger and thus a bit more difficult to get mixed into the sand. It is still a problem.

1997 Lego spill - https://en.wikipedia.org/wiki/1997_Lego_spill

The Cornish beaches where Lego keeps washing up - https://www.bbc.com/news/magazine-28367198

After 25 Years at Sea, Shipwrecked Lego Pieces Are Still Washing Ashore on Beaches in England - https://www.smithsonianmag.com/smart-news/whimsical-legos-ar...