Richard (and others),
First, the personal experience. I tried using festoon LED bulbs in my masthead mounted Perko Anchor/Steaming light (2 bulbs in the one fixture). I did this despite what I will talk about next, due to the fact that this fixture is a good 10-12 above the deck and therefore difficult to replace bulbs. The LED bulbs did not last. I replaced them at least twice, and both times they stopped working(one time in only about 20 hours of use the other about 1/2 season). Regular bulbs last for at least one, often 2 seasons, so I am back to them. I looked into buying a new LED (designed) fixture, but it is very expensive compared to the same "regular" fixture, and I am not convinced that the bulbs in the designed fixture will last longer. I think it is the moisture that causes LED failure in this exterior fixture??
As far as I know, Leo and others have the "rules" correct. If you replace navigation/anchor regular bulbs with LED's, your light is NO LONGER IS COMPLIANT. Yes, lots of people do this, and most "get away" with it.
I know of one court case (in the US) where an intoxicated boater under way at night collided with an anchored boat that had it's anchor light on. The moving boater's defence lawyer spent 2 full days at the court proceeding challenging whether or not the light met all certifications, regulations, requirements, etc. Lots of "expert witnesses", etc. Also, insurance companies have been known for finding reasons for denying coverage.
It's your boat, you take the risk (or not), you decide. For me, I decided for several reasons (especially the bulbs did not last and I could replace one LED bulb with incandescent about 4-5 times for the same price, and it seemed the regular lasted longer as well). The legal issue was secondary, but certainly something to consider and be well aware of. By the way, these were not "cheap" LED bulbs, they were from a reputable supplier.