I have a 1997 seadoo speedster boat

[marionr26]

I notice that it always stays on 1/2 tank how much does it usually cost to repair the gas gauge I think this is a valuble tool...Does anyone ever keep a extra gallon gas with them on boat???? I feel up like each time out dont usually ride but a few hrs not even sure how long can ride on a tank..Only been out twice since getting this boat end last yr. I also try adding oil in it each time out...It has a oil sensor light not sure it works though hard to tell how much oil is in there since its a long hole down where oil goes...This has the twin 85hp 2 cycle rotax engines on it......Any response will be grateful...It has two knobs at seat that turns on and off the gas do I need to turn these off when through riding for the day or can i just leave them on...thanks