That "non-return valve", a.k.a. "one way valve" or "backflow preventer" is something some people put in to help a toilet keep its prime. In my thinking, it is most useful for trailerable boats, boats in high-low slips, and those that otherwise get hauled frequently, etc.
It can also be useful for faster or planing boats when the raw water intake is located forward. But, well, I'd recommend against such a location.
Personally, I think it is just one more thing to break, so unless the the boat was out of the water a lot, or the type of boat and installation called for one...I wouldn't put it in.
Having said that, if "something ain't broke, (I) don't fix it". So, if a prior owner did something many years ago, and it has worked for many years, I don't rethink it -- I just replace it as it was.