At the end of the gameplay trailer of the newest game, Monster Hunter Online, we see a scene of a Plesioth being chased by an Airship and 4 WaterMotors. Extrapolating from the information, we can assume that these 4 are Hunters with the Airship in support.
From studying the scene a few hints about their construction can be formed.
First is that these ships appear to be much larger than a real world Jet Ski. They each are equipped with what appears to be a ballista or a harpoon gun. They also have a heightened wheel-deck/bridge that has a standard sailing ship's wheel for control and 2 lanterns for light at night. Their also seems to be 2 fins on the sides towards the rear of the ship that may be used in steering or stabilization. Just under the waterline at the rear of the ship appears to be the device that gives it the quick propulsion speeds as seen in the video.
Taking these aspects and our current knowledge of the Monster Hunter universe, we can piece together a rough idea on how such a craft may operate.
It is noted that at the current time, the Monster Hunter universe is in the beginnings of an industrial revolution with the development of steam engines. This seems to be the likely source of power for these WaterMotors. It is not uncommon to use steam as a source of power in ships, in real life, most large naval and cargo ships are powered off of steam, even the large aircraft carriers of the US Navy (even though the heat source is a nuclear reactor). Steam engines are considered to be external combustion engines, that is, the fuel burring is outside of the working force (the steam).
For Monster Hunter, the common source of this external combustion is a fire fueled by wood or coal. For these WaterMotors, these common fuels may not be the heat source; instead it may come from something like a Flame Sac from a fire wyvern. Once the water becomes steam, their needs to be a way to transfer the steam's pressure energy into mechanical energy. There are two main ways to accomplish this. First, one can use a steam piston (fig2). These are typically able to deliver a great amount of power for low repetition. This would be useful if the WaterMotor's propulsion was from a waterwheel or some sort of tail like device that required such power.
Personally, I feel that this is not the type of propulsion used. Those forms of propulsion tend to move slower and thus can't provide the speeds seen by these ships. The other way to transfer energy from steam to mechanical is a far more likely candidate. The steam turbine (fig3) uses rotating fan blades to produce a large amount of RPM's with relatively lower power. This is the system commonly used in real life ships and can easily be attached to a propeller or impeller. These propulsion sources are more likely to produce the high speeds as seen in the clip.
All of the mechanics I suspect are housed in the enlarged housing at the aft of the ship.
The technology used in these WaterMotors seems to indicate that a lot of thought and ingenuity has gone towards furthering the capabilities of hunters