E Online quotes Tatum as telling Jimmy Kimmel Live that the pair opted to go for a swim one evening while taking time off from the shoot.
Tatum said: "We decided to jump in the ocean - naked. It was night-time, thank God.
"There's Jerry's Deli right on the corner and [Jonah] decides he wants to go in and all he's got is his T-shirt and his underwear on, and he's like, 'I'm going in to get something to eat!' and I'm like, 'No!' So I stopped him and [took] him back to the car."
He added: "Now he's nicknamed 'Baby Jonah' because I had to dress him like a mum. I'm like, 'No, no. Bad Jonah. No. Butt up, butt up. Okay'."
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> '21 Jump Street' review
Watch a trailer for 21 Jump Street below: