In honour of the last day of Shark Week on the Discovery channel! I'll start.
Group of 18/19 year olds skip school and pinch one of their dads' jeeps. Dad is the town mayor and had a terrible experience in the water that he just doesn't talk about, but his free-spirited daughter is NOT allowed in the ocean. She, of course, ignores this and they drive to the beach awhooping and ahollering and lots of 'this is going to be the best day everrrrr guys'. They bring loads of beer with them too and have a party on the beach.
One of them suddenly remembers about a cool cave nearby that is only accessible when the tide is out and talks the others into investigating ...