Step 1: Setting up
I would recommend a 5x5 field because filling 25 tubes is a reasonable task.
Step 2: Filling the tubes
1x fishing boat 1 tube
2x patrol boat 2 tubes
1x cruiser 3 tubes
fill these with a liquid with a pH higher than 8 ( soapy water)
that leaves 17 tubes which can be filled with highly diluted vinegar.
Step 3: Ammo
this will cause a correct guess to turn blue and an incorrect one red.