I've been watching this string with interest, as I have a '75 170. It's never had an electric pump, probably because of the hassle of running a hidden discharge line under the floor.
1. While not a very sexy solution, I carry a long, high-volume hand pump on board. It moves a surprising amount of water per stroke...perhaps a gallon up/down (I should check this). Not a huge amount but perhaps 200 gph at a 'do-able' pace. (That's ACTUAL volume, by the way)
Pros: No battery, no wiring, no switch, no maintenance, moves some water. Will suck water down to about 1/4" deep. Completely dependable unless I have a coronary, in which case, 'who cares', right? Fits under my console very nicely. Cheap.
Cons: No battery (tiring after a bit); requires opening the pie plate and looking in the bilge occasionally (can't just flip a switch and wait to see a stream of water); people think it's amateur which is sorta true but hey, my bilge gets dry.
Before anyone reacts, I'm not suggesting that 'manual' is the equivalent of 'electric'...only that it does get the water out, down to a remaining volume that a sponge can finish off. I like the idea of having a nicely installed electric too.
2. About the remaining water in the bilge. A small additional pump in the aft bilge would probably get you down to 1/2" of left over water (no float...just a manual switch). Perhaps just add a tee and use the same discharge line?
As has been mentioned before, a big pump is desirable on a small boat...50 gallons of water is a pretty big percentage in a 175, compared to a 22-2. But it pulls more power so you better have enough battery. Also, you'd be surprised how often float switches get stuck "on" and drain a battery. Per the Harbormaster, it happens all the time in our marina.
