Given the voltage drop at the pump, yesterday evening’s project was to wire the pump with 8 AWG (10mm sq) for the positive feed, and a bit of 2 AWG (35mm sq) for the negative feed.
Unsurprisingly, the pump worked very happily, and the clamp ammeter gave me glimpse of ~10 – 11 amps of max draw; looked like inrush current.
I’m not sure I’d recommend my cabling technique, however.
“Clamping” the 10mm sq to the battery 
The pump wire is inserted into the 35mm sq cable, and taped in place. That’s the meter probe stuck through the cable. 
Clamp showing the current
That little UNI-T meter is very nice; the clamp feature is useful for things like this, and the auto-ranging voltmeter and ohmmeter capabilities are neat. It’s a UT210E.
Dad looked up the values in a boat electrics book (I have one too, but it’s packed away in a box somewhere), and we concluded that for the 6 metres of cabling (two 3 metre legs) over 10 AWG / 5mm sq should be fine (if not overkill).
We also checked the max draw over the feed to the wiring locker, and concluded that the 20+ amps that get pulled over the 7 metres of cable are too high for the cable size, so it’ll need replacing. I can replace this cable first, with 6 AWG, and see if the pump gets happier. If it doesn’t get happy enough, I’ll do the dedicated run via the battery compartment instead of via the wiring locker.