To be fair though Ron the pacemaker would be in contact with fluids continuously. With a model boat there would only be contact with water for short periods of time and coating with epoxy resin is a tried and tested method.
I had a similar situation with painting my Faireys having decided to brush paint them. I decided on International Paints as for full-size craft but it is not recommended for use below the waterline even though it's an oil based paint. Having put several coats on it has produced a wonderful and hard finish and I've got no concerns about it being water resistant for the time the models will be on the water.
Back to coating the inside. If I was building a sailing yacht, especially if plank on frame, as they tend to get water inside and there are nooks and crannies, I'd epoxy resin it. For boats with good access and ply sheeted I'm happy to use Eze-Kote, even though it's water based, as any water ingress can be easily mopped up. Others may disagree though?