Punt versus punt. Often with people fighting on the banks also, since attacking the punts from there is advantageous. The punts are then used to ferry people from one bank's battle to the other. So this do be a game with lucrative desert islands. This is all totally hopeless unless there's resurrection every couple of minutes, since being on a boat or being trapped against the cam if not result in swift death.
We eventually decreed that being dead did not stop one from punting or paddling, just from firing. Without that, the footbridges in particular are impassible.
That way it is a boat race as well as a multi-fronted battle.
Another tip is for the punts to have dozens of litres of water each. Camwater can damage guns and is not a very nice substance to be shot with. Though people do occasionally fall overboard!
We often take a picnic with us for half time. This is kept in a properly dry compartment.
Serious rules: no shooting at or from bridges with traffic or on college property. The upper Cam is far better for this (from the Mill toward Grantchester).
Note: there are no bridges between the end of Lammas Land and Grantchester, so we don't usually go much further upstream than that. Helps keep us together if (usually dead) people can move between banks without needing to summon a pirate ship each time!
This event is traditionally held at the end of Mayweek.