fantasy

it takes place on an island at first i remember and they use magic songs and there is a tale about some monster siren creature and i remember them walking up a tower a lot of water and possibly a few caves in the story it might bee a magic school and the kids are sneaking off for some reason

sorry it has been about 10 years since i read this book and desperately want to find it again

Some overlap with Sea Hearts (also published as The Brides of Rollrock Island) by Margo Lanagan, but that might be a stretch.

Possibly Song quest and sequels. Set on an island at a school where youngsters learn to sing magic. A lot of the action takes place at sea.