Matanzas is a beautiful place, a great spot for kitesurfing and windsurfing. great waves, good winds. favourite spot for a lot of pros. be careful with the rocks. it´s a spot not recomended for beginers or intermediate´s.
you are in the south pacific, so take your full wetsuit 4/3 recomended at least. to be more confortable use a 5/4mm and boots. all year.
this spot works better in the warm month´s (september to april). if there are good conditions on a weekend it can be crowed, so be RESPECTFUL and be careful. Kiteboarders respect the windsurfers.
North of Matanzas, there are other points, like Las Brisas, La Boca, etc, good advice.