YA book with a spooky lake

Heya I’m looking for a book I read around 2014-2015? But it seemed a little older (pages were yellow and worn from my primary school library)

I vaguely remember details but I’ll try to be thorough

- it was mainly the POV of a young boy, I think either his uncle or grandpa would travel around with his inventions, Da Vinci was mentioned once or twice

- he was in love with this young girl about his age who was his best friend since they were children. They made a blood pact of some kind to be best friends forever or get married later (they do get married later in the story)

- pretty sure the kids were fairly young 12+ at most 16-17

- I think it took place in Italy? I think I vaguely remember Italian words being used in the book.

- weird detail but I know the girl could play the recorder really well.

- the story was mostly centred around this lake in the town I’m pretty sure the lake was haunted one way or another. I do remember there maybe being a ghost (a woman???) that they have to face off? Maybe

Sorry it’s a lot of maybes but I’ve been thinking about this book a lot lately and since it’s a tad obscure, I’d like to find it! Any help is appreciated :)