There are three main things that you should look for with solar panels.
First, are the cells made from mono-crystaline or poly-crystaline wafers? You can tell by looking at them. If you do not see any grains on the cell, then it is mono. If you see multiple cyrstals then it is multi. Mono's are more efficient and produce more power from the same area. They are also more expensive.
Second, how rigid are the panels? With thin, brittle solar cells you do not want the panel to flex too much. An average size person can stand on a well built residential panel causing no damage.
Third, the achilles heel of PV solar panels... the electrical connection. If there are connectors see if they are water proof. If there is a junction box see if it is sealed. All the same problems you have with your other electrical connections on your boat will be the same with the solar panel. If the connection corrodes you may not be able to fix the panel without destroying it.
One bit of interesting information about solar panels. The different brands all use the same wafers and cells. There is quite a bit of "horse trading" in the industry. The major differences between panel manufacturers is the panel construction as mentioned above. Many of the different brands of panels are made in the same factory at contract manufacturers. So, don't get hung up on brand names. Look at the warrenties and prices.
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