Sweet and nice to hear you have enough power without needing to add panels.
5mph is a nice speed for cruising.
lithium batteries are expensive, I have bought 5 or 6 laptop batteries and opened them up to retrieve the 6 to 9 18650 batteries that are in them.
I bought my battery packs on fleaBay
So I have about 40 batteries at 2000 - 3000mah each
which cost me about $50 , they take up about the space as a red brick.
It will cost you a lot of money to use lipo or lithium batteries for the canoe , a lot of money - For enough lithium's to match the spacial area of a 12v car battery it would cost $500-$750 if you found cheap batteries, but it would be a mass of Power