There really are not many components that go into a solar panel. From a rigidity standpoint it all has to do with the frame and how it supports the cells. That is easy to see.
From a waterproof standpoint you really have to look at the laminate that encloses the cells. Ask around about how different manufacturers last. Once water gets in there the panels are done.
From a power standpoint, keep in mind that the panel ratings are peak watts in ideal conditions. You can get a rough gauge of the efficiency of a panel by the total area of the cells compared to the peak watts. You will see several methods to maximize the captured sunlight such as different colors of backing material, anti-reflective glass, the anti-reflective coating on the cells (that's the blueish color you see), and the collection line pattern.
I have an older 40Wp panel that was made for the Canadian CG that is almost bullet proof. It is not large, but fits my needs. It is mounted on my sea hood over my companionway hatch. I can stand on it with no ill effects. I need to watch out for falling winch handles, hail, and any stray shackles.
All panels will be severely effected by shade due to the combination of series and parallel wiring needed to get the proper voltage from the panel.
Look at what the warranty covers. Panel warranties should be at least 20 years. Make sure salt spray is covered though they should all be fine in that environment if they are a quality panel.
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