The boat I use most often gets about 150 MPGe on 8 210 amp hour 6 volt golf cart batteries 3 phase permanent magnet dc motor with 2 belts driving a prop shaft out the keel.
Cruise speed is 5 mph Flank speed is 6 mph, realistic range is 70 miles. It’s a 1982 Duffy electric packet.
Edit: here we are on our local river.
https://youtu.be/rm2FhL8X4yU