Hmm, you might want to think that through a little bit.
This is one area where you may be better off working up to it in stages. You might want to start with a stationary engine just running on compressed air.
Building a steam powerboat is more than just building the engine. You will also have to build a boiler and fuel system, plus the controls that go along with it. I'm not trying to discourage you. Just letting you know what you are in for.
Firstly, boilers are NOT toys. It is not a task to be taken lightly. Improperly built or operated they can be very dangerous. Even deadly. You will probably spend more time and money building a safe and proper boiler than you will on the engine itself.
Search for and read some of the posts about boiler safety. There are also a couple build threads. In particular Firebird has built a very nice boiler. His build thread starts
HERE
After you have studied all you can on boiler building, operating and safety by all means go ahead and enjoy every minute of it. Post your progress here so we can all see. We like to see successful builds.
As far as engines, a twin wobbler is about as simple as you can get and have plenty of power, especially under steam. Try one of
Elmer's Engines. You may have to make modifications to make it drive a boat, but they are good designs.
There are many knowledgeable people on this board. They are a wonderful resource. Make use of them, ask lots of questions.