Here is a link to a Homebuilt ROV hobbyist (http://www.homebuiltrovs.com/howstarted.html) They have built several units using both bilge pumps and also they have a design for a homebuilt magnetically coupled thruster. Right now, the OpenROV uses brushless motors. Some have modified their OpenROV’s to use more powerful commercial thruster units.
As far as “power” goes, the power comes from both the combination of the prop and the motor’s ability to turn the prop. There are a number of websites available on-line that deal with optimum prop design for specific applications, marine engineering, etc… The good folks at OpenROV have done some experimenting and testing of various combinations of motors and thrusters and I believe they have come up with a great setup for Hobbyist built micro Observation Class units.
If you are dealing with small ROV design then many components (motors, props, controllers, etc…) are readily available through Hobby stores that specialize in radio controlled boat models.
ROV thrusters (as with all thruster applications) are typically designed to provide sufficient thrust for operations in a particular environment. For example, are you operating in fresh or salt water, what depth, what degree of control you need and will there be significant flow (current) in the water column.
More often than not small observation class ROV’s are low to moderate powered because they are likely to operate in areas that do not have swift moving water or strong current. However, there are some applications where a small ROV would have need of a more powerful thruster combination. More powerful thrusters often cost much more money so most times people build to suit their intended operational need. Think of it like this, If you had an old VW car and intended to only run it around town and at low to moderate speeds then it is likely that you would equip it with an inexpensive stock four cylinder motor (most economical option). But you almost surely would not spend thousands of dollars more to equip it with a 500 hp race motor, unless you intend to race it.
Another consideration is power supply. This factor will also impact what motors can be used for thruster system. Are you using onboard batteries or surface supplied power? Will you be running a 10 to 12 volt system, 18v, or something significantly higher…
Then there is the issue of motor control. If your ROV is going to be digitally controlled by onboard computer then you will need a motor that is compatable with available ESC.
These are just a few considerations when selecting the best fit thruster for your project.