Where to see wild sea turtles from the mainland?

I have heard they are commonly sighted on Vieques, Culebra, and Mona islands. I have also seen vague mention of them being found on the "northern beaches of the mainland." I would be most interested in that, as to avoid booking ferries and planes to/from the other islands. Where and how can we best find them on mainland Puerto Rico (without being too intrusive, of course)?