I don't know if anyone has investigated this, but a few things that younger (even the very young) are interested in could be combined/integrated into a model boat.
Robitics, Rasberrypi/Arduino, and drones. Before anyone mentions costs, the amount some kids (or their parents) spend on drones, tablets and apps is not small.
In the same wat that people move from model aircraft to model boats (stress, crash costs, etc.), people could start to move from flying drone racing (with FPV) to drone boat racing with FPV.
I also see a lot of Sim APPs for phones based on transport, truck driving, truck parking. Perhaps modifiying Springer Tugs (a very cheap platform)
to drones and have course running competitions, and parking/manouvering loads. Or for the real adrenaline junkies, making drones of Club 500 boats.
If you look at the popularity of robots, a lot of the attraction is in the design and build, and the programming etc. Water would also provide new challenges for roboteers.
Just some thoughts.