Perhaps trying to define common sense rules seems somewhat inane. Be that as it may, I still think it's worth a shot and may prove useful to some.
1. Read, respect ,and follow all game rules
2. Safety first (i.e. no harmful additives in water supplies, no point-blank face shots, no throwing rocks, no use of real weaponry, etc.)
3. Respect all players and non-participants
4. No aggressive or abusive physical contact. Optimally, only water streams and/or water balloons should be the only source of contact between opposing players.
5. Respect personal and private property. (Do not destroy vegetation, do not use hoses not part of the game, etc.)
6. Respect game boundaries. (i.e. Do not go beyond the playing field, do not enter forbidden zones, etc.)
7. Respect equipment (i.e. Do not play with triggers, free pumps should not be left extended when not in use, etc.)
8. Respect game time. Attacking before and after games is prohibited, as well as attacking eliminated or respawning players, or non-players.
9. Show honor. When you are clearly the one who was hit first, do not complain and leave the field. If uncertain, be willing to have a replay or call it a draw. Disputes should be settled by either having both players go out (which is the typical method), or having a sudden death type match if both players are the last ones remaining)
Anything else should be added? Anything need more clarifying or re-ordering? Should anything be removed? Thoughts or ideas most welcome!