you can find out if they are structural, its not a perfect test and you would need the owners permission to do it. lucky its not really a destructive test and it takes only a few mins to do. like i said its not perfect but it can show up cracks that are just more than gel coat.
get some sand paper grits 80 and 150, then you need some sort of dye, what works well is red kool aid.
first lightly sand the crack, not a real large area, infact using the edge of a folded piece is best. apply some koolaid mixed strong, wait about 1 min then wipe clean, then using the edge of a piece of 80 grit folded sand the crack some more, basicly opening it to a V just till you hit the fiber glass or until the line goes away. as you sand watch and wipe it out every so often. if the fine line that the koolaid drew into goes away it is just the gel coat. if it does not go away it goes deeper. if it does go away you can double check it buy putting more koolaid in to the V let it sit then wipe with a damp rag. hopefully it all stays one solid shade of pink if the line returns then it does go deeper.
if all is okay sand again with the 150 folded to get rid of the stain, and use gel coat filler to fill up the V ( optional if you own or are going to buy the boat )
this is a trick i learned when i used to run remote control submarines, and i built a run of 9 foot long fiber glass hulls