DO NOT, jump a seadoo battery while the engine of the car is running mate! you run the risk of frying your whole mpem in your seadoo which will run you a hefty replacement fee. you can use a battery pack to help you jump, or a non running car. although when i tried this when my ski died ( with a non running car ) it would not work or make the ski start. i eventually bought a trickle charger for 20 bucks at canadian tire.. oh wait, your from nz sorry bro, any car parts store, they should sell a powersports trickle charger that plugs into the wall, i cant imagine it being more than 30 bucks, get that.. its saved me many sleepless nights.
hope this helps! have a good one bro! :cheers: