Okay, I'll add my own.
This boy, Danny, ends up somewhere by the coast on holiday. The groundskeeper is always whistling Danny Boy, which is how I know that's what his name is. Danny makes a friend with one of the locals, and there's some sort of hidden treasure and a triangle, I think? Maybe a celtic knot triangle? Anyway, I remember a part where they're in a river in low tide and have to hurry up to get something finished because the tide's going to come in, but the groundskeeper wants the treasure. Something about a church, maybe?
It's stumped everyone so far.
There was another one that I'm wishing I could remember what it was. Girl visits coast town with a bay. Ghost ship appears in the bay. Girl and friend try to figure out why ghost ship is there, it's drained all water out of the bay. Girl's family bought a tavern, I think, and behind the wall is lots of money for girl's family that they now get to keep, they bought the tavern. It was behind a mermaid in the wall, I believe.
I need to start writing down all the books I read, ever. Really.