i'm studying your designs magmafry and the others on the wiki
btw, since water has 7+1=8 levels, i'm thinking maybe a septagesimal or octal computer would be possible
but it seems inefficient in my head, though it would theoretically make the design compact.
i'll probably be back with more on this as i read on
I don't think that you can use water levels as efficient data storage, not only because 1/7 water will evaporate, but also because you can't quickly adjust the water level without having to disperse water over several tiles, creating a need to wait for unknown time.
Also dispersion over multiple tiles will give the water an opportunity to evaporate, which does not work well with your design principle.
The only way to avoid these consequences is to not distribute water at all during the calculation process, by using differently leveled water tiles that you do not allow to combine under any circumstances, thus effectively rendering the water level useless, as the potential paths of differently leveled water tiles cannot combine without elaborate security measures to prevent the water itself from combining, in which case you could just use 7/7 tiles for the same purpose.
Also, if you want to accurately read the water level of a water tile to an accuracy of 1/7, you will need at least 4 different pressure plates and an elaborate mechanism to differ between the different results, which is actually worse than just using 3 bits of 0/7 or 7/7 water.
Final note: All my designs mentioned here still work perfectly if you replace the 7/7 water with 6/7 or 2/7 or stuff, but only because all pieces of water have strictly separate paths.
Other final note: a septagesimal system has base 70, which is probably not what you meant.