When I lived in Haiti for a few months I would go visit the beach of Petit Goave and it was remarkably beautiful. The best part about it was that it was very open and undeveloped and it was easy to pitch a tent and sleep literally feet from the waves. There were also little coves where the waves were not too harsh, so you didn't have to worry about getting swept away in your sleep.;)
It is easy to get to Petit Goave on a bus or other form of public transportation from larger towns like Leogane, and not even too far a drive from Port-au-Prince. If you want to do something less "rustic" there is a nice beach restaurant. Otherwise, you can give a local fisherman a few bucks and they will go catch you and cook you food right there.