If those rivets go through the window and into the fiberglass hull, the only way to remove the window for re-seal will be to drill the rivet.
We drilled the heads off and then "pushed" the shaft of the rivet to the inside with a small punch or awl. I don't remember any rivets in our windows
other than the door window. All of the other rivets in the windows
were structural to the window frame, not attachment.
If it's possible, have someone stand by on the outside (to play window catcher) while you remove the window frame on the inside. After you get the inside frame off, carefully pull back the inside wall covering around the window to see if those rivets go through the hull. If not, then all you should have to do is pull the window out.
Next step is clean all mounting surfaces, apply butyl tape, and then re-install. seal around the outside of window with something like flexi-seal if desired. There is a great thread on the board here about re-sealing with butyl tape, and the optional bike tube/rubber gaskets. Per Walthinsen had some great pics located in this thread here
, it's allmost a tutorial.