Broken or lost paddle; or boater is tired (losing stability, strength, confidence) - possibly has fallen in and remounted but subsequent fall outs and remounts will become increasingly fatiguing and will compound difficulty of rescue.
If a fellow paddler has lost their paddle, attach your tow rope (see in safety gear section) and paddle them to safety. If you don't have a tow rope, or the other paddler needs more stabilization in rough water, pull your boat up next to theirs and put your foot in their footwell. Each person paddles on their outside of the surfski raft formed by the two skis. Split your paddle in half if you need to. An even more stable “raft” can be formed if possible by 2 rescue surfskis, one on each side of the rescue-ee. Then the paddler in the middle being rescued simply helps hold the outside skis close (if they are able to do that) while the outside rescuers, each with a foot inside the middle surfski, paddle on the outside of their surfskis.
If a paddler is in the water and can't fully remount, get them half out of the water so they are either lying across your boat or theirs. This can add valuable minutes (of slower heat loss) while you wait for help.