I've found a list of 6 spots that are supposedly the Fishing holes on the Diadem islands.

Southern Diadem Lake (8 / 30)
Secluded Diadem Pond
Northern Diadem Lake
Blustery Cloudtop (12 / 25)
Calm Cloudtop (33 / 13)
Swirling Cloudtop (28 / 33)

Still missing coordinates for the 2nd / 3rd ones, and a map would be a bonus, anyone?