Hi, Jim
Worry not! You are doing the right thing, and all will be well.
Your original post was about a fuse rating. Generally its a good idea to fuse a system capable of high power - and Lipos certainly come into this category.
The motor you have will take only a nominal power if you don't fit a prop!
Neither the motor or the battery govern to power required - hence the fuse rating - its the prop that does that
All the power figures come from aircraft use where there is a near-infinite range range of propellers available to load the motor exactly as you choose.
So: I didn't know the boat till now - I see that the motor is fitted and it seems to have an outdrive, possibly surface-piercing prop. The motor (as martinno1fan) said will be spinning about 17000 rpm (if the prop allows) and you will get to know it (remembering Hailwood's law "the throttle goes both ways".
If it were mine I would just try it as it comes, and feel the motor after a 5 minute run. I suspect it will not be warm in the slightest (with 3 cells). Probably a 50A fuse would do the job - that would suppose about 500W power. that's half a kW and roughly 3/4 horsepower. Sounds about right!
I see that the standard blade car fuses seem to be available up to 60A, with in-line holders to suit. Suggest you get a small selection from 30A to as big as you can find and see if you can blow one. If you have a fuse which is spot on for the running power it will probably blow if you hold the boat and give it full welly. So go up a size till it doesn't! (note - if the speed controller has a very soft start it might not blow on this test) You actually
want it to blow if the prop gets snarled to protect everything else.
Just for info - low voltage fuses go up to about 1000A - they are called fork-lift fuses and are fitted to fuse base - can be home-made. I have seen (and used in my Espace) car fuses up to 175A - there is a very neat one available from RS which fits over an M8 bolt and has a ceramic body -
http://uk.rs-online.com/web/p/automotive-fuses/3377993/I hope this gives you the confidence to proceed - its for fun
Andrew