I was a little concerned because of arthritis in the shoulders/arms, but this was a wonderful experience. Perhaps we were lucky, as that night there were 5 Mantas feeding under our lights. These are very big creatures 5-12 feet across. They have learned to swim a looping manoeuvre to best catch the plankton in the light beam. So with the magnification of the water/mask interface, they seem to come inches away from you at the top of their loop. No danger they only eat plankton. Some found the rocking of the raft straining on the shoulders & a few got nauseated, but EVERYONE thought it was well worth it.