This is the perfect way to spend your day, and the cheaper way to go to the beach when you don't have a car.
-Do not forget to check the schedules!!!
Here you catch the boat than when you get to Trafaria just ask how to get to the beach (Praias de São João). Sometimes there are some people who will give you a ride. When you enter the car park (with the gate) just keep walking and turn right at the end of the road. Then walk as many as you want and choose the beach you prefer. The most populated are the first ones, and "Bicho de Água" (one of the beaches names) is the most populated one so avoid that one.
Av. Brasília, 1300-598 Lisboa, Portugal