I have heard of a movie about this... part trailer but didn't watch it cause it was broadcasted late at night.
The yacht didn't have any ladder for the swimmers to be able to get on the yacht again (and the yacht had high 'walls' if I can call the outer side of the boat like that), hence they were forced to stay in the water, and eventually died. Is that it?
Well, initially, all the people jump in the water to go swimming, but nobody thought about seeing if they had a way back to the yacht.