Generally, a tank of water for a water injection system is about half a US gallon in size and is supposed to last for about as long as a tank of gasoline does, or about 300 miles. The reason for this is because underhood space in automobiles is limited on one hand, and probably to remind the driver to refill the water tank when he fuels up on the other hand. However, on a boat I would think you could use any size tank that was practical to install and connect it to the pump for the water injection. If you decide to try water injection, one other thing that you will probably want to do is to add a bit of denatured alcohol to the water, not to add any more power, but to retard algae growth in the water tank. I would think that a 5 gallon gas can or water can would work pretty well for a water injection tank and would give you several hours' worth of running time. That's just a wild ass guess on my part, but any water injection system that you buy should come with directions that tell you number of untits per hour that the system will consume. And I suppose as a last resort if there isn't information about water consumption included in the instructions, call the company and ask them.
I also am a firm believer in the KISS system, but I do think that using a Pertronix ignition system instead of points would be a good idea. Again, it doesn't cost a whole lot, has one moving part, is all contained under the distributor cap, and uses a magnetic Hall-effect trigger to fire the coil, which is about as foolproof as you can get with electronic ignitions. I have one of these on a 1968 Mustang, and I love it. Takes about 15 minutes to install and never needs adjustment after that. Hope some of this helps give you some ideas or answer questions.
