This Game is meant for Boy Scouts.
Decide for yourself if it is appropriate for your younger scouts or not.

many 5ft lengths of strong twine or rope (at least 3 per patrol)
3 5-6ft staffs or bamboo per patrol (broom handle size)
1 mousetrap per patrol
various items to retrieve - neckerchief, plastic bottle, baseball cap, ...

Drill a hole or screw in a screw-eye to the 'safe' end of each mousetrap.

Mark the river bank, past which scouts may not step.
Place the treasures about 12 feet out from the bank.
Patrols can use as many lengths of twine as they need to lash their poles together to create a fishing pole, tie the mousetrap to the end, then use it to catch items and retrieve them.

You may have the same set of items for each patrol to retrieve and the first done, wins.
Or, different items may have different values that you can tell before the contest or after.