After replacing the Spektrum mr200 with a mr3000 I didn't really see much improvement. The next thing to do might be to move the receiver to a location far from power lines and try to find a better way to mount the antennas. At the same time I will probably replace the Arduino with a non-defective one so I can program it.
On the good side I have found a way to mount the windows that I like. I will probably need to get another photo etch set for the window frames though. Another good thing: I didn't see any water ingress from the propeller shafts. Furthermore a guy from the model boat club has a spare part for the transom mounted plate that I lost earlier. Also good.