Froggio,
3/16th's is pretty much the standard diameter of the pop rivets used for gunnels on boats I've seen. What size drill bit did you use to drill out the original rivets? That's the size rivet diameter you need. Length of the rivets needed would depend on if it's a thin section boat hull of fiberglass or a thicker section of Royalex, polyethylene, or some other thicker hull material. Measure by sticking a nail or piece of wire through a hole and measure the portion hidden. Average several holes so you're sure the one you picked first isn't a fluke.
I'd use aluminum rivets, though other materials are available (stainless steel?). The latter would be way overkill. I'm not sure what you mean about "grip." You'd want long enough rivets to go through all the way to the inside of the boat plus maybe a quarter inch showing inside when the rivet head is all the way against the outer gunnel. Make sure as you start with each rivet and when squeezing them until they "pop" that the head of each rivet is all the way against the outer gunnel or you'll skin knuckles on that one later (don't ask me how I know). Drill out and replace any with heads that aren't tight flush.
Good luck with the project.
