Propshaft and Propeller

This is perhaps the most obvious choice and sounds straightforward.  Get yourself some props with propellers and attach them to your motors.  In my experience it's very difficult to get them aligned correctly which causes vibration and in the case of using two motors can mean that at full power, the boat will tend to veer left or right.  You will need a few bits and pieces such as propshafts, propellers and brackets for keeping the motor secured.  Also you'll need couplings to connect the motor to the prop.  Best place I have found to look for these items (in the UK at least) is Cornwall Model Boats (although I am not connected to them in any way).  Link to propshafts here

 

Note:  Spinning propellers have a habit of attracting weed and fishing line which causes them to foul.  You can take steps to put weed gaurds (mesh) over them to counter this risk but it is something you should consider for your own project.

 

Jet Propulsion

My latest boat uses a jet system (similar to the ones used for JetSki's) which I believe is less snaggy and much easier to set up and align.  Some of these items are eye wateringly expensive but I am currently using the one on the left which you can get from ebay (albeit Hong Kong) for less than £10 (link here)  If the link no longer works go to ebay and search for 'NQD 757-6024'.  It comes with a propeller inside and a coupler to attach it to a motor (although it's probably the wrong size). 

 

Additionally it also has a kind of rudder (basically a movable collar on the output spout) that would allow steering if required.  If you're going to use two motors for steering, these collars can be easily removed and discarded.