How about some real life experience, and I'm an ET to boot. Without doing the math etc I can tell you a little battery like that on a tiny boat will give the grandkids about 4 hours of fun. I have an 8 foot porta bote that weighs about 60 lbs. With one person it really scoots with a 30 lb thrust motor. Using one as you want to should easily give you 4 hours total on the lowest setting.
If solar would drop in price and become flexible one could visualize the top of a kayak covered with cells. One day soon I hope.