I try to stay out of controversies like this because like motor oil
threads it usually turns into a no-win situation.
The issue with exit pupils and low light performance is one of those things. As mentioned, if you take the objective diameter and divide it by the magnification you get the diameter of the image that is transferred to the ocular side. The theory being that in darkness, our pupils dilate to 7mm and therefore we get an optimal match between the image and the opening that our eye gives to our retina. One of the issues is that our eyes can take several minutes to several hours to adapt to darkness. The pupils can dilate rather quickly but that is not the entire story.
The other factor is that greater magnification can provide greater detail that we can perceive, even with a smaller exit pupil. This is called the "twilight factor" or "twilight coefficient" of a pair of binoculars. It provides a better "real world" comparison on how different models can actually perform on a functional basis and is a (possibly) slightly better number for choosing a pair for nighttime use.
However... not all binocular use is at night anyway, and during the daytime the pupil constricts to about 2mm. Now a pair or 7X50's will generally give you a larger field of view, but not always (it depends on the design) but they are heavier which can make a difference in holding them still for an extended period of time on a moving boat.
The one thing about optics, is if you compare a $50-$100 pair of binoculars to a $250 pair of binocs by looking though one then the other, it's an OMG! moment in terms of clarity, and the bad news is if you then compare that $250 pair to a $600-$700 pair you get the same reaction again. And I am one cheap
guy, so for me to say this takes some doing (just ask my wife). Also, the pricier optics have better lens coatings which improve light transmission
One thing about optics is they seem to take a pretty big depreciation hit, so if you do not mind purchasing
used there are some bargains on ebay and a pair of binocs that sold for upwards of 5-600 can be bought for a couple hundred but if you want to purchase
new optics I would suggest going to one of the larger camera
stores and talk to a knowledgeable salesperson who can help you make an informed decision (and not simply some advice from some nameless guy typing stuff out on the internet
, like me for example!).
Best of luck in whatever you choose!