Crabsign,
The water is just not pure enough to use, even though it might look like perfectly clear water, it will be full of suspended solids, and even though they can be filtered out, some will get thru and build up a deposit in the boiler, plus when you sail in different areas, you don't know whether the local water is hard or soft. Unlike full sized steam plants, these small ones really do need to be looked after correctly, otherwise small orifices and supply lines can get blocked up.
I was lucky with my steam engined boats, I live in a very soft water area, so could use just normal tap water, without the risk of scale build up, but normally, people buy distilled water or collect rainwater (correctly filtered of course) for running their engines on.
MCR,
That is a wonderful installation and hull you have there, it brings back lots of memories for me. If looked after, you have a model that will steam for a lifetime.
Bogs