It sound like enormous fun but unlikely to be safe or insurable like people have said, what about giant catapults and water balloons filled with coloured dye?
http://www.thepresentfinder.co.uk/buy/giant-water-balloon-catapult-warning-for-16-only_127.htm
Might be tricky to operate from a small vessel, or even have a small target on each boat that the other crews have to take a properly centred photo of (imagine that would present a challenge in the rolling seas) count the photo's at the end and impose some kind of time penalty for each "direct hit".
http://www.thepresentfinder.co.uk/buy/giant-water-balloon-catapult-warning-for-16-only_127.htm
Might be tricky to operate from a small vessel, or even have a small target on each boat that the other crews have to take a properly centred photo of (imagine that would present a challenge in the rolling seas) count the photo's at the end and impose some kind of time penalty for each "direct hit".