Remember that umpteen thousand lbs of "slippy" yacht or a fibreglass fishing dinghy is a vastly different drag factor to a few hundred lbs of transom dragging rib in displacement mode.
Then you need to carry enough battery power to keep it going....... Their quoted 3-4 hrs running time on an 80Ahr has the disclaimer "we presuppose you do not drive all the time on full speed." I'd check that statement out in a bit more depth for a start, because your 55Lbs thrust will be at full chat, and based on running my rib with a 4Hp petrol burner, to get more than a couple of knots, its not far off WOT.
VERY rough calcs: say 3 hrs on an 80AH, that's (80/3)*12 = 320W in 1 hr. Using a random online converter, 320W = 0.429Hp
The only "truly eco friendly" aux is a setr of oars.