Haven't had my head in one of those boats enough to know it well. BUT... behind the helm on most seadoo boats... there is an open power lead to run extra gauges. Have you looked behind the dash yet?
As Dr Honda indicated, there is a spare 12 volt plug in the helm of your Sportster. Remove the bin and you will be able to see the auxilary plug. Tap into the wires and you are good to go. It is already fused.