Unfortunately, you have posted to the Tips & Tricks sub-forum, which is basically a repository for articles on how to do things. Maybe our over-worked MOD, Steve Grisetti can move this question out to the main PrE forum, so that many more users will see it, and can give you suggestions.
Now, let's look at your Audio footage, and what you want to do.
The section of ski footage is noisy with boat engine noise. How can I reduce just that section of noise. I don't wish to eliminate it just lower the volume.
Success depends on exactly what you have in the way of audio signals, and exactly what you want to do with it.
If there are other signals in the Clip, that you wish to keep, and only wish to lower the Levels of the noise floor (the boat's engine), you might have success, but that depends on two things: the frequencies of the sound that you wish to retain at original Levels, and the frequencies of the boat's engine. I believe that PrE has some limited EQ (Equalizer) Effects. I normally use an audio-editing program, with a full compliment of EQ's.
I'd start with a Graphic EQ Effect, and then spend a lot of time adjusting the sliders for each frequency band and doing careful listening. It is likely that you can attenuate (lower) the Levels for some of the frequency bands, that ONLY contain the signal from the engine. You will loose those same frequencies in the ambient sounds, but there is no way to separate those out, and address them individually. Depending on the number of frequency bands that your EQ offers, this might be able to isolate many of the changes to the offending frequencies. Do not know how many frequency bands the Graphic EQ in PrE will offer, as I have never explored any.
In Audition, one can also sample a part of the file, with just engine noise, and then work to eliminate most of that from the whole Clip. Note, one still has a balancing act, of lowering the Levels across the frequency bands. I would go lightly here, and then work with the Graphic EQ to clean things up a bit more. Last, I'd look at both the High-pass and Low-pass filters, plus a Notch Filter.
Since I noticed that you had also posted to the article on noise reduction, I know that you have read it. However, when your post gets moved out to the main forum, and others drop by, I will link to that ARTICLE here.
Now, and so that others can perhaps give you specific ideas, can you tell us what part of the audio spectrum you wish to keep at the original Levels? Also, if you have a short piece of the Audio, can you attach it as a PCM/WAV? Hint: use the little Attach Files (paperclip) at the lower-left of the forum editing screen. Let us know exactly what you'd like to try to keep. We'd be happy to process that small section and see if we can make improvements to it, and then post the steps that we took to arrive at our edited piece.