It's not clear what "multiple broken handles" exactly means, but I am willing to bet that what's actually wrong is not that the whacking great steel handle itself has broken but rather that the pop rivets have pulled out of the fiberglass. In that case you really should fix it right, not just pop-rivet a new handle down to the broken fiberglass. In other words, go get a boat repair kit and patch the fiberglass, then when you go to rivet the handles back on, don't use the puny little backup washers shown, but rather make a backup plate out of steel or brass sheet, to spread the load.