Check the colour of the flag on the life guards' post. A green flag means that you can go swimming in all safety. When you see a yellow flag, you are allowed to go swimming, but you should bear in mind that it can be dangerous. Therefore it is forbidden to take rubber boats, rubber rings or other inflatable or floating objects with you in the water. A red flag signifies that it is strictly forbidden to swim.Furthermore, you should follow the life guards' instructions at all times.
Surfers have their own green (surfing is allowed) and red (surfing is forbidden) flag, easy to recognize by the white logo in the shape of a surfboard.
Keep in mind that it is forbidden to swim in unguarded areas! These areas are indicated by a prohibition sign. Going swimming nearby breakwaters and moles can be very dangerus because of the strong currents and whirlpools.
Find the different flags and signs here.