A man pretends to be rich, and takes 8 friends on a large boat that belongs to someone else, they get into the water and after a while, realize no one can get back on to the boat.

Maybe you are thinking of Open Water 2: Adrift

"Six friends go out on a large boat to have some fun and halfway through the cruise they decide to go for a swim. Once in the water they realize that they forgot to put the ladder down, which means they are stuck in the water."

Thank you very much greatly appreciated.