They haven't changed much in body shape and they're tough. They can go a long time without eating, survive extremes of temperature and salinity. They can survive for days in fresh water or very saline water, many hours of high temperatures which would kill other sea creatures, and apparently, high doses of radiation. They love clams, but also eat worms, dead animals and may even scrape off algae. They can also force open small bivalves.
Horseshoes generally creep slowly over the seafloor munching along.
But they can swim for short distances above the surface, using their swimmeret legs and gill flaps. Horseshoes breathe well in low-oxygen muddy water. They have 5 pairs of flap-like appendages to which are attached an array of book gills, as many as 200 of them. The horseshoe waves these flaps around to get oxygen. They can even use these gills to swim upside-down, but apparently only juveniles do this.