Waverunner sat for 4 months. thought battery was

Try jumping the waverunner using a set of jumper cables and your vehicle, this will rule out if the battery is bad. If you still have the same issue, check the connections. The connection at the starter or another place may have corroded a bit. Yet another option is to remove your spark plugs to make sure that your motor has not hydraulically locked. Once you have removed the plugs try and turn over the engine. You may need to ground out your plug wires if you have an electronic ignition to protect it. I hope this helps.