Paul,
I've found that reduction gear noise is relative. If things are aligned correctly there will be some noise, if not aligned correctly, lots of noise. Noise, to some arbitrary point, makes the boat sound more realistic (to some folks, at least), and unless it's really excessive, won't be heard from any great distance (that comming from an ol'fart who'z half deaf so take with a dose of salt).
The motors being overkill? Just depends on if you plan to tow/push anything. If you do, then they are not overkill, never have enough power. The 'trick' is training your self to keep your 'foot' off the gas! LOL (I do fair at that, but can't say it isn't fun to chase the ducks!)
The 'belt' reduction drives should be/are much quieter than the metal gear ones. I wish I could find them at a reasonable price (shipping etc. Only source I've been able to find isn't in this country, rats!). There again, no actual hands on experience so take with that 'salt' mentioned earlier.
I've only found/built one model for which the instructions were 'good'. The rest range from 'bad' to "Wonder why they bothered putting this @#$% on paper?". I think part of the reasoning (?) is that you're supposed to take your time, understand what the particular procedure is all about before doing it, and that the 'fun' is in the building (then, there's the part about they just didn't take the time to do it [the instructions] right). For the multi-language instructions, think of it as a free lesson in learning a new language (just don't try to ask the where'a'bouts of a bathroom in that language when you finish the model - LOL). You can 'justify' almost anything if you can stretch far enough...
Good luck and have 'fun'...!
- 'Doc