Balboa Island has twelve notes to find.
Follow the path signs to relay station's 4 and 6. Go to relay station 4 first and get the two notes that are inside of the station.
Relay station 6, which is the station you should go to next, will also have two notes.
Around the base of the cliff that relay station 6 is on top of, you will find a trail of notes leading to the campfire where you get the Tiny Little Murderer achievement. There are five notes to be found in this area.
Lastly, you will go to relay station 2 and the ranger's station. There is one note inside of station 2.
There are two notes inside of the ranger's station, however you can only get the one behind Bruno's board after finding all of his tools as part of the A Revelation achievement. *See "A Revelation!" guide* for more info.