At 10 and 12 I bought my kids a 14' aluminum boat with a 9.9 Tohatsu on the back. The ran this for two seasons. They would tag along and tie up with us or go explore the shores and creeks. I would let them wander Cumberland when we were out on the water. They stayed in radio contact and had a dog for supervision. Never got into any trouble and they were very safety conscious. After two seasons I upgraded them to a 18' ski boat with a 302. I would allow them to go as far as the radio would reach and they would check in every hour. They have always followed safety rules and never given me cause to worry. I let them run my boat on occasion but never more then 5 to 10 minutes away and they are not allowed to get into boost unless I am on board.
Only give kids as much responsibility as then can handle and they will be fine. They do most of their own maintenance and chip in for gas. What could be better than to be 12 or 14 and have your own boat with a thumpin stereo, full tank and ski pylon?