YA coming-of-age rowing island boy has a tutor and catches father having an affair Spoiler ahead

[A]

A teen like to go rowing with his father, and one day they find a small island with a cabin. The father buys the island and cabin. The teen has a tutor and one days catches his father and her having an affair in the cabin The teen has to deal with the consequences of his father's infidelity.
Book from the 90s. 1993?

[Rainbowheart]

This sounds super familiar to me.

Was it set in modern times?