Which figure of speech is used in this text?
Sandra, please stay away from the ocean. You shouldn't go in the water until you know how to swim.

 - euphemism
 - paradox
The text uses a paradox, a statement that might at first appear to be contradictory, but that may in fact contain some truth.
You shouldn't go in the water until you know how to swim at first appears to be contradictory, because it is impossible to learn how to swim without going in the water. However, it contains some truth: you should not go into deep or dangerous water without first knowing how to swim.