For someone without the dual use I have (my airplanes run on either 3S or 6S packs) I'd certainly go the 14.8V power/garden tool battery route too. Much simpler with an added safety factor.
My system will be the lightest weight (because airplane batteries are built that way) with the downside of some additional monitoring during use. My tube's fish finder has a voltage display option I always use for a fuel gauge anyway.
And I already have the proper chargers for the airplanes too.
I have some ideas to experiment with much lighter weight brushless DC motors that should dramatically improve range with greater efficiency.
Where there is an option to use a boat ramp near the fishing hole, the heavier equipment has the clear advantages.
I can see your advantage of having the cells available for multiple applications. I also can use in multiple apps, from cordless tools to electric bikes, skate boards, mowers, etc. , but not RC aircraft. One thing to watch out for is not to let your lithium batteries get submersed or soaked from rain or splashes.
The only present advantage of heavier electric propulsion equipment is $$ cost that I can see, if everything else is the same. I think lightweight electric tube propulsion of proper design can do some things that heavier electric cannot, but not the opposite. Also one can go to gas motors which have greater range and are faster but have other issues, plus cost more.
Troll motors are built for moving heavy bass boats and are very inefficient for powering tubes, is what I have learned. Brushless motors will help, but are not as important as hydrodynamics, according to what I have learned from monitoring the boat design forums over many years.
Hope this helps.