Surface Marker Buoys are essential for freedivers in open water to inform other water users where you are even when you're submerged. SMBs for freedivers are usually permanently inflated unlike scuba divers dSMBs that are inflated underwater to mark their position. Apnea Buoys differ from scuba buoys also by having handles and attachment points used to attach lines and accessories but can also be used to rest in the water. Larger buoys can have pockets to stow fresh water or spares so you don't have to return to shore.