Dead in the Water by Tom Turner and Serena Turner

While Alexa and Clay have nothing but passion in mind, twelve cutthroat killers board her billionaire father’s super yacht.

Clay, an elite Delta Force captain, is home on leave and has no plans except to spend every minute with his girlfriend, Alexa.
Then the gang of ruthless pirates board her father's yacht and find a handful of fat cats and a harem of good-time girls in full party mode.
The killers have but one demand: that Alexa deliver three hundred million to the boat or her father's a dead man. To prove they’re not screwing around, the body count starts to rise.
Faced with no better alternative, Clay sneaks on board in a desperate attempt to save those on the ship. His chances— you guessed it—slim to none.