The 18-year-old found the net a total of 26 times during his outings for the Under-18s and Under-21s, which resulted in him being named the Academy Player of the Year.
Discussing the striker's development, McCarthy has conceded that he has been "delighted" with what he has seen.
"It would have been great to include him in the squad had we been safe with three or four games to go, but we weren't," McCarthy is quoted as saying by the East Anglian Daily Times. "He's doing very well Jack. He scores his goals, he's industrious. I've been delighted with him in the Under-21s."
Marriott was recently handed a professional deal by the club, which will start at the beginning of next season.