You have lost or broken your paddle, cannot effectively paddle for some reason, or cannot stay upright in your surfski.
If conditions are not too rough, you may be able to lie on top of your surfski and hand paddle.
Turning your surfski upside down and lying prone, straddling the bottom side up with your weight low and head down can be more stable on some skis than lying prone on an upright surfski, providing you the ability to make adequate progress back to shore.
Which way to turn your ski and lie on is better for you and your ski? Is this technique at all practical for you and your surfski? You will have no way of knowing unless you have tried it, preferably with controlled, safe tests (close to shore, on shore wind, clothed for the weather and water, help readily available if needed, etc.) in rough enough conditions.
Even if you can make little progress paddling using this method, lying on top of your surfski will allow you to stay warmer than swimming will. That may make a critical difference by allowing you to stave off hypothermia until rescuers find you.
If you are proficient with this kind of paddling you may want to also consider carrying hand paddles as backups.