My question does it matter if I use open or closed back rivets? Closed-end rivets are supposed to be "more" watertight.
Similar to: A casserole dish is "more" watertight than a colander.
Closed end blind rivets are a type of tubular rivet with a mandrel through the center. To install a closed end rivet, the entire fastener is inserted through drilled holes in the parts being joined, and a special tool is used to draw the mandrel into the rivet. The blind end of the rivet expands to secure the workpieces, and the mandrel snaps off. Unlike open end blind rivets, closed end blind rivets provide 100% mandrel retention. The head of the mandrel stays inside the set rivet, and won’t find its way into your assembly where it could cause electrical shorts or other problems. Closed end rivets block the passage of fluids and gases to create a tight, secure seal.
Open end blind rivets do not seal out gases or liquids.
If replacing a factory rivet through the original holes, I'd use 1/4" diameter and ream the holes, if necessary.