Lagoon it's true I rode the waves back in high school
A lagoon is a shallow body of water separated from a larger body of water by barrier islands or reefs, the double entendre in this line could be spotted without this detail, but it provides great information.
He could be saying that he was “shallow” but he was “overshadowed by a bigger body”, just like a lagoon so he just rode the “waves” and did whatever was cool to do.
He could be saying that he rocked actual wavy hair back in high school.
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