Sentence: Come on people; we haven't got all day. It'll be time to get out...

[Hongdung]

Can you help me to explain this sentence:
Mr. Bob said: "Come on people; we haven't got all day. It'll be time to get out again before you set foot in the water."

(Mr.Bob was a swimming instructor in a college; he was speaking to pupils at the swimming pool; but I don't understand what he means when he said : It'll be time to get out again before you set foot in the water.)
Thank you!
Jung

[Mister Micawber]

.
The class time will be finished by the time you finally get into the water.
.
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Native English teacher at Mister Micawber's