Why was this rejected?

Examine images at 100-300% magnfication.  That is your image.

For legal reasons, I cannot use your image for my commercial project because you did not submit model releases.  Or to put it another way, if I tried to use your image for a commercial project without signed model releases, I would lose my job. Adobe would be sued.  And you would likely be sued for not having model releases.  Do you see how that works?  Model release are required to protect everyone, including you.

How do you deal with the questition of model release when the subject is silhouetted and middle distance, and unidentifiable... is not know to the photographer, and is in a small traditional fishing boat where the destination is unknown?

One of the comments indicated that model releases would be needed from the two people in the boat in this photo!

Are there specific guidelines available to guide us? Maybe a link?

You need to give us the refusal reason.

I see several problems:

First, it's out of focus. The people are not sharp, they should be.

Second, it's underexposed. Looking into the histogram, you'll see that there are no whites:

The people are even more underexposed, you should lighten the shadows here.

As a last, you will have a model release for the 2 people in the picture. If you do not have, there is no need to try to work the picture. If you have, you will need to do an outstanding job, sharpening and correcting the exposure, without introducing too many artefacts and enhancing the noise level.