All of the engine sounds are represented by 4 different mixes:
Mix 1 is a mono recording with the advantage of the engine over the propeller.
Mix 2 is also mono, but with engine rumble hidden in propeller splashes.
Mix 3 is the stereo one, with the additional track mixed in. It enhances the water splashes with much wider recording.
Mix 4 is also stereo, but the engine level is lower, so the water splashes from the back of the boat could stand out in the mix. Naturally there are RAW files, so you can design the sound on your own. Last but not least, additional files recorded on the boat while floating and running. You can read the full description of the files in sound list here.