I grew up near mountain lakes, and the OR Coast. At 14 I likely knew everything, and was foolish!
We did some stupid things, and I'm not going into it, but kids are going to use poor judgment. For example that moon remark, that's great, until something blows in and covers the moon! :mooner:
You can get them a cell phone, and put it in a Pelican case. Some will float, but you'd have to do a bit of research, and determine if it's going to survive you 14 year old. There's also the will he use it part. When I was young I likely would have skipped calling mom only to report that I'm breaking the rules.
A flashlight is never a bad idea. Heck, you really should consider marine flares, and gear designed to make boats safer. Of course flares are dangerous, but they could be valuable in an emergancy.
Red/Green lights on the front MIGHT help other boats avoid them assuming you can find something that would run off a battery, and can be mounted. An emergency light is likely much more visible then a low end weak dive light. Something like
THIS
I do think home BEFORE dark is the rule to enforce. If he won't follow it, take away the privilege for a week. This is not a kill joy, this is being a parent!
Unfortunately he WILL break the rule. Another thought is to gang up on him by having both you and the other boys parents enforcing the same set of rules. I always got into more trouble when my rules were stricter than the other kids as it just resulted in me getting home late.