I grabbed some bits off eBay to experiment with:
This 12V pump is sold (eBay under $5) as a BMW replacement part.
It works well, but like most windscreen wash motors, is damn noisy.
Interestingly the polarity (not marked) doesn’t seem to make any difference.
This little submersible pump is rated for 3V – 6V operation.
It comes with a cable tail approx 100mm long.
This one operates quite well and is nice and quiet.
It was less than $2 (free postage) on eBay.
Maximum lift: 40-110cm. Flow rate: 80-120L/H.
Water outlet: 7.5mm O/D, 5mm I/D.
Diameter 24mm, length 45mm, height 30mm
This one was around $5 with free postage on eBay.
It drew water quite well with about 300mm of hose on the inlet side.
Although rated for 12V use, it does work down to 5V or so.
Operation is reasonably quiet once the pump has primed.
Rated Voltage : DC 12V
Load Current : 0.3A
Max Flow : 2-3L/Min
Outlet Pumping head:1-2.5Kg : 1-25M
Water Hole Dia : 9.2mm/0.36″”
Total Size : 85 x 40 x 32mm (L*W*H)