I'd venture to say you're going to need quite a bank of cells to get enough amperage to accomplish any meaningful charge progress. For the batteries many of us use in bass boats, a recommended charge rate capacity is easily 10 amps - for best health of properly charging the battery. Ten amps at 13.5 volts is 135 watts. The solar cell on Amazon provides 5 watts. That's possibly OK to maintain a charged battery (but not charge a depleted one).
If you had enough cells to provide several amps, you'd still be able to charge only one battery at a time. I see no reasonably viable solar cell option for what we need to charge boat batteries.... My solution was an enclosed storage bay with power - until I owned a home with ample garage space.