Steam engines, and boilers are an art unto themselves.
They are not usually cheap. And cheap ones will not run with
the power you would need to go out, rescue a boat and bring it back.

The other issue, is that the boat will need to be kept steamed up in order
to be able to perform a rescue the instant something happens.
The next most likely occurance, is that when a boat does need rescuing, the
steam boat, have been keeping steam, and waiting,... will need to be refueled, and
need a fresh tank of water... which will need to heat up and produce steam...

This might all take two to five minutes, but if a boat is floating off, you may need that
time to catch it before it gets away.



However here are some links that might give you some ideas...

http://www.jensensteamengines.com/


These would probably have the power you require... [8D]
http://www.monahansteammodels.com/