I won't tire of saying it and you will find similar tips all around my Bretagne pages. The water here is cristal clear. It is also much less salted than in places like the Mediterranean but unfortunately colder.
Plougonvelin is a small town along the coast, at the entry to the bay of Brest. Located inside its own small bay, Plougonvelin offers some of the most beautiful and protected beaches in the area.