David, it depends on what fixture you have. Some boats came with Aqua Signal, some with Forespar. Take a picture and check it with binoculars, and compare it to the ones in the WM catalog, which will also show what lamps (bulbs) are used. Good luck.

If it's an Aqua Signal it likely uses a 10 watt mini-bayonet Halogen bulb, Ancor has them.

On related note, the quality of Aqua Signal lights seems to have tanked. I bought a new deck/steaming light two years ago to replace the original 15 year old unit. They changed the steaming light bulb from the dimpled end festoon to a pointed end festoon which seems a mistake, it never worked reliably from day one. Wiggle the bulb, it works for a few days then quits. I tried bending the copper holders to increase tension, made no difference at all. The deck light is not much better, the bulb will not sit in the holder properly, when you change it and close the housing it gets knocked out. I am replacing the $105 Aqua Signal unit with a $30 made in China knock off job that oddly enough uses the old school dimpled bulbs. If it only lasts a year it's 100% better than the "genuine" article that didn't make it a month.

Bud; Aqua signal has a series 25 and a series 41 - not sure which one you got... Why not go with the Forespar?

All the stuff on our boat is Series 25. I have all these spare dimpled end festoon bulbs, LEDs, and 10 watt Halogen deck bulbs for the Series 25 which I would like to be able to use. Not sure what the Forespar uses but these bulbs fit all the nav lights that I currently have.

I'm just really tired of paying big bucks for junk equipment. The first Series 25 light on this boat lasted from 1995 to 2011, it still worked but was corroded and unreliable and past it's best before date. The brand new replacement, that looks the same (except for the stupid pointed end festoon holder) lasted about 4 days, and this isn't even a salt water boat! Clearly they're doing some serious corner cutting somewhere.