Get him to the level of a good swimmer for his age and then take him snorkeling.

My son loved swimming with the fish at that age.

Fishing and preparing the fish could be fun.

He can help drive the dinghy.

You should have a blast.

Warren S/V Scuba Doo
Lagoon 50 (winner of best crewed yacht under 55 feet at the St. Thomas crewed yacht boat show).

https://www.cyabrochure.com/ebn/2307/pdyrX/6642/4///