We are mostly looking for something cheap with a water-cooled motor. If we order from Amazon, it's pretty rare that you can take a look inside to see if things are modular (not an integrated electronic speed control (ESC) or servo). We have plenty of ESCs and servos in our workshop so we didn't care much. But if you don't have a ton of spare parts lying around, you may consider making the trip to a local hobby shop to look at the inner workings of a boat before you buy. Here's the boat we picked up (as shown in the following image):
It moves at up to 15 mph (far faster than our duck will swim), but this means that it'll have plenty of power to move our (much heavier) duck at a speed of around 3-4 mph. It may even reach 10 mph. We'll never drive it at that speed, but we like to follow Clarence's rule—it's better to have something and not need it, than to need it and not have it. This rule got its name from the movie True Romance. In it, the character "Clarence" says...