These split shaft rivets are used to install handles and accessories in locations where there is no internal access. They are particularly useful for installing extra drag handles onto skis. They can also be used to secure hatches and pad eyes/saddles when rigging your kayak. Since they open into a star when installed they are much stronger than a standard pop rivet and will not pull through the fibreglass easily.
They do have a hollow shaft however and should not be used below the waterline. A dab of silicone or sika 291 is also recommended to seal the rivet shaft after installation.