The only time I watercooled an esc was with an Electronize 15 Amp type. It was running 2 off Speed 600 motors and total current14 amps (measured under load).
I made a plate to fit the base of the unit with 2 off 5mm dia tubes soldered on and joined at the top by a neoprene tube.
I had a simple water scoop and a return for the water. The return was flush with the boat bottom and sloped away diagonally so the exit hole had a larger area then the scoop.
The scoop and exit part were cut from plastic oval section electric cable conduit about 20mm across.
The esc ran stone cold at 14 amps and the design leant itself to an easy flow of water for cooling.
So although this is all old hat regarding esc's you can run without a pump and you do not need much contact area for the cooling side.
For brushed motors the heat is around the brush area.
All the details were published in International Marine Modeller for the build of the Graupner Bremen 9. She is 40 inches long. The build photos I took were before digital photography so I found this one on the Internet.
Hope this helps,