Man from Atlantis (1977)

After found washed ashore, Mark Harris is believed to be the lone survivor of the legendary continent of Atlantis. Having adapted to life underwater, Mark assists an institute for undersea research operated by his human friends, Dr. Elizabeth Merrill and C.W. Crawford, and joins Dr. Merrill in several exploratory missions aboard a high-tech submarine. They encounter several bizarre phenomena, including portals leading to other dimensions, a substance capable of altering personalities, an impish creature whose touch causes a mental return to childhood, and the scheme of a portly millionaire, Mr. Schubert, to melt the polar icecaps.