There is no way I would have a tank with the pickup pipe above the lowest point of the fuel. I reckon any contamination needs to be sucked out as it happens and caught in a filter. Not for me the potential of having 20lts of gloop in the bottom of my tank. A big problem waiting to happen.
Any reasonably designed tank would have a fitment inside, similar to a swirl chamber, which holds fuel at the pickup pipe while the rest of the fuel is slopping about. For a boat, to me, it makes no sense to have otherwise.