This movie was released in 1985, so it’s pretty old.
From Wikipedia, we have the aliens disguising themselves as humans:
About 10,000 years ago, peaceful aliens from the planet Antarea set up
an outpost on the planet Earth, on an island later known to mankind as
Atlantis. When Atlantis sank, twenty aliens were left behind, kept
alive in large rock-like cocoons at the bottom of the Atlantic Ocean.
Now a group of Antareans have returned to Earth to collect them.
Disguising themselves as humans, they rent a house with a swimming
pool, and charge the water with “life force” to give the cocooned
Antareans energy to survive the trip home. They charter a boat from a
local captain named Jack, who helps them retrieve the cocoons. Jack
likes Kitty, a beautiful woman from the team who chartered his boat.
When he spies on her while she undresses in her cabin, Jack is shocked
when he discovers she is an alien. After the aliens reveal themselves
to him and explain what’s going on, he decides to help them.
And the people swimming in it and becoming youthful:
Three of its residents, Ben, Arthur, and Joe, often trespass to swim
in the pool next door, thinking the house to be unoccupied. They
absorb some of the life force, making them feel younger and stronger.
Eventually caught in the act, they are given permission to use the
pool by the Antarean leader, Walter, on the condition that they do not
touch the cocoons or tell anybody else about it. Rejuvenated with
youthful energy, the three men begin to let the advantages of the pool
take hold as they are relieved of their ailments (such as Joe’s cancer
miraculously disappearing and Ben’s vision becoming clear, allowing
him to be able to drive a car once again).