Don't assume anything when it comes to the electrical system of these things. Just because it was on a maintainer, does not make it good. If you have the resources available, put a vehicle beside your ski, then run jumper cables from the battery known to be good, to your jet ski. MAKE SURE YOUR VEHICLE ENGINE IS OFF, then try to start your ski from the vehicle battery.
Getting two beeps and lighting up your gages, takes a tiny amount of power. The starter motor is the one most power hungry piece of equipment on any motor, due to their high torque.
Your other choices are to remove the battery and take it to "Advanced Auto" or another parts house, and have the battery load tested. In general, these batteries only last one to 3 years. If your in a cold climate, like New York, the life span is twice as short than in Florida, unless you bring it inside a climate controlled area, such as a garage. Cold is the number one killer of batteries. Even if you got a maintainer on it, if your still storing this battery outdoors, chances are it's dead.