I'm a bit late responding to this thread as I've only just joined the site. My main interest is R/C aircraft and we have the same problem attracting younger members.
The two main factors that seem to affect us are the need for our flying sites to be more remote than boating lakes and the need for parents to transport their youngsters to site and stay for maybe 4 hours doing something they may not be personally interested in. Access is also a problem with our site in bad weather (all last year it seems) as we have to cross two fields to get to our strip – can be very muddy. This may not be such a problem with boating/lakes.
Another factor which I assume also applies to boating is the need for parents/guardians to be present to comply with the child safety legislations.
Here in West Sussex access to water suitable for boating is limited as angling clubs seem to get priority (money talking??). Our local council has limited access to the water in our countryside park from what I gather even though there is room for all users.
John