I do not have first hand knowledge of what they are used for. I'd be inclined to first ask someone at Sam's to view your photos.

Unless you get a definitive answer, I'd leave them alone. In wood boats such triangular supports were often used to reinforce stern to bottom or hull side to bottom.
Here are a few suggestions: Try holding a magnet close to them..they appear to be rusted ferrous (magnetic) material but I cannot see them clearly enough to be sure. Another option is to wire brush one and see what metal is under. If not magnetic, then for sure leave them alone as a stainless material will last a long, long time. There's a good chance this will last another 20 years anyway.

What's on the other end of these fasteners?? Either they are bolts and the other end must be accessible for a change out, or they are lag bolts (a screw thread) which is less likely. If something was bolted together prior to installation, perhaps by glass encapsulation, you'll not likely be able to get them out anyway...

good luck.