Nolan Wells repeatedly refused to leave Horn Island on his buddy's boat ... at least according to his best friend.

Warren Hudson went on "The Officer Tatum Show," where he told Brandon Tatum that Nolan was repeatedly asked to get on their boat before they took off.

The way Warren tells it, it's not uncommon to go to Horn Island on one boat and come back to the mainland on a completely different one.

He said his buddy was "panicking" trying to get Nolan to leave with them, allegedly saying ... "Nolan, get the f*** on the boat."

Warren claimed that Nolan wasn't the only person in their group to leave with someone else that day.

As you know, Nolan's body was found in the water off Horn Island, days after he went missing on the Fourth of July.

His friends reported he'd stayed behind to hang out with a girl ... but that girl told police he was planning to head back with his pals.

Nolan's family is conducting an independent investigation into the teen's mysterious death.