Hard to believe and I wish you would prove me wrong but the units I have been investigating do not show water flow in their documents. Dometic, MarinAire, Flagship. Nothing online, I have downloaded their user manuals...nothing. Everything else but that.. Hope you prove me wrong but I can't find them and have searched. I find it very odd as well and keep thinking I am just missing.
All three units are no more than 3' above the waterline, the pump is center line about 2' below the waterline. going to confirm today but I believe the 16,000BTU and 9,000BTU are within a 10' hose run from the pump. The 6,000BTU unit, in the v-berth, is about 15'. I may be underestimating these a little but not much. I have no idea of where the length turns into an issue for me. Maybe those lengths are well within reasonableness, not sure. Will also confirm today but I don't believe there are any 90 degree turns in the lines. There are two 90 degree turns to get the water into the manifold right after the pump. After that, all are pretty simple runs.
I can understand the "each with it's own pump" thinking but these are not crucial to the boat and the thought of having 3 through-hulls and 3 pumps instead of 1/1 seems to present more potential issues than is necessary.
Anyway...it sounds like the pump I have is adequate for the new units and if I wanted to replace it, or have a spare, the March pump mentioned would be adequate at 15-17 gpm.
I know it may seem like I am "obsessing" but I am well aware that I don't know what I don't know. Just trying to learn.