I've never done anything that large, but it sounds like fun - get away from little fiddly bits on the 1/8 scale. 10-15 is over the size the launch folks typically usem they like to keep the boilers smaller than you need for something that size. 7hp seems more typical and in that range there's the Elliot Bay and Reliable Steam. I spent some time talking to a guy a couple of years ago at NAMES who was making a nice triple from his own castings, so something like that is always an option.
You could start smaller - Tiny Power has some nice engines in the 5HP range that I've seen on launches. Here's one of their smaller engines I made:
(I've since remade the badly done steam chest cover)
And a little twin:
I've never done much with boats - there's not too much water in New Mexico and most of it is a long drive. I used to do RC and made a couple of sailboats, a rowboat (really!) and once a hovercraft. My only attempt at an electric boat was a miserable failure and after retrieving it in late January I stopped working on it.