You are right, the original engines are 6V. I bought mine at about 1966. I still have it but it never was on water with RC activated. At the time I bought it, the boat was NOT expensive, but the neccessary RC equipment was it. The boat costed about 60 German Marks, the small parts set, engines, accus, colors etc were about another 60 Marks. A usefull RC-set would have a much higher price well above 1000 Marks, I did not have that money than.
Right now the boat is ready to go to sea, some years ago I bought a cheap RC Control set and a friend put it all together last month. Unfortunately my RC is crap, so I had to buy a better one from Graupner called Hott mx-12. I will switch to this soon. After all the boat has a real chance of getting to its destiny after just 46 years.
By the way, your boat has a minor fault. the fog horn belongs on top of the lower roof, just in front of the middle window. Otherwise it looks like the original idea of the boat. Mine did never have the underwater color, it was grren but now I switched all visible decor lines to bright red. I will use a 6V Pb-Accu with it, though I also have NiMH powerpacks.
I will take pictures of it and put them in here, when I have the time for it.