MUSA, the Museo Subacuatico de Arte, is one of the strangest and most memorable outings from Isla Mujeres: a museum of sculptures installed underwater where art, reef habitat, and snorkelling tourism overlap. The pieces are meant to be seen through water, light, and marine growth.
You need a boat tour, snorkel trip, glass-bottom boat, or dive operator to visit the installations, so conditions matter. At its best it is both playful and ecological, a good alternative to treating the reef only as scenery.