Using any type of "recycled" Li format in a battery build is simply a bad idea regardless of the use. The inherent issue with Li formats is that once they reach full or empty the voltage does a hockey stick. Over or under charging will murder a cell. We call these two points the upper knee and lower knee. When you place cells of unknown capacities & internal resistances into a bank keeping the cells in balance will be near impossible.
Of course the only Li format that really should be used on a boat would be LiFePO4. A typical 18650 LiFePO4 cylindrical cell runs from about .8Ah to 1.5Ah (beware of any 18650 claiming more than about 1.25Ah).
In order to build a 100Ah 12V nominal battery you would would need to start with about 600 used 18650 LiFePO4 cells. You would then need the test equipment, with high accuracy, to test each and every battery/cell and pick the ones that match closest. On a 1100 mAh cell for example a 2-3% variance in Ah capacity would be a 1067 mAh cell on the low side and 1133 mAh on the high side.
After multiple months of cell matching you'll now need to begin construction. It would take 4 100Ah 3.2V cells to built a 12V nominal 100Ah LiFePO4 battery from 18650 cells. Each of the four cells will require 91 18650 cells in parallel. This means that you will need 364 near perfectly matched 18650 cells to build a 100Ah battery. If you were to buy these new you're looking at roughly $5.00 - $6.00 each for legitimate quality LiFePo4 18650 cells such as non-counterfeit A123/LithiumWerks cells. So on the low end you're looking at over $1800 just in cells to go 18650...
How will you then attach 91 cells to the cell end boards? Solder? Machine screws? You can buy cells with end studs but they get pricey. What will you use for the end boards? Copper? Tin plated copper?
Course a far better solution would be to go with large format prismatic cells, such as CALB, Sinopoly or Winston, where a 3.2V 100Ah cell will run about $130.00 or 4 X $130.00 = $520.00. You can't even buy the test equipment needed to test hundreds of used 18650's for $520.00....
So now we have an assembled 12V 100Ah battery and we now need a BMS, contactors, thermal monitoring a re-wire to the vessel, a battery case etc. etc........... It goes on and on...
