Another issue with the bayonet bulbs with single
center contacts is when you put the bulb in and slide it, the friction on the bottom turns the little piece of copper on the bottom that the bottom center of the light contacts to. In many cases, they only solder a lead to that and isolate it from the ground. A lot of turning will eventually breakdown the strands of the wire and/or solder joint.
Long story short, it is often easier when you replace bulbs to also replace at least the bayonet mount and if there is a lot of corrosion, suspect the seal is broken.
Do not pay an electrician. You can get through this yourself, even if you have to buy a new aqua signal light, it will be less than having an electrician doing a boat call.