Cardboard boats sail and compete until defeat
Cardboard Kayak/Canoe/Boat Part 1/2, shallow water, floats/ balls
See activity "Cardboard Kayak/Canoe/Boat Part 1/2" and bring said items on a warm/nice evening.
Locate and use a small shallow water location eg a canal 2-6ft depth boating area perhaps near a lock which normally has road access. Make sure you have appropriate water safety helpers since risks are relatively high.
I would suggest YP wearing buoyancy aids but definitely check the swimming capabilities of YP since they will probably go in the water.
Canoe paddles would help but can be improvised.
Plan to have competitions between boating teams, some ideas...
1. Solo across canal/distance timed. (~10m)
2. Dance and be daft on craft.
3. Quickest over short course. (~10m)
4. Most amount of floating objects picked up in ~5m square.
5. Balls on water to be thrown at another boat winning with hits means points in ~5m square
6. All get on craft to show capability, oh dear it will sink! (Help may be required, if good, to sink)
~5m square competition area can be setup with four simple floats, made with 2l milk carton, ~3m rope & brick with holes.
I would suggest running another activity alongside this so the crew of each boat gets a chance to demonstrate the full capabilities of their craft until they need to extract the soggy remains and place back on a trailer to return to the hut for drying and disposal.