Just tossing this out there; its your event obviously so you can decide... But what about this?
Min. 1 Boat
Max 3 Boats
Min ppl 2 per boat 1 operator 1 gunner
Max 5
that will allow a total max of 15 people per kingdom to participate
for the 5 people
1 to operate the boat
2 for the guns
1 Archer
1 Repair man
or 2 archers
Overall though I really do love your Idea
If the time is reasonable Ill participate :3!