Sail out to this amazing petite prison-fortress constructed in the 16th century by Vauban, on a small islet in the Bay of Morlaix. Most departures are from Kelenn beach in Carantec (15km northwest of Morlaix), while a few leave from the port of Diben in Plougasnou (17km north of Morlaix). Check online for schedules.
A joint ticket with the Cairn of Barnenez gets you a reduced fee for both sights.