I had the exact boat and all the fluorescent fixtures work on the boat's 12 volt circuit.
If I'm not mistaken, all replacement bulbs are the same. They will work on either a 12V or 110V circuit. When they are put in a 110V fixture, the ballast drops the voltage down to 12V. The few times I bought replacement bulbs, I got them at the hardware, not a marine or RV store.
Sounds like you've got a bad switch.