Surfski.info Safety Gear Article 2019 - read this. It is based on extensive experience.
David Morrison's personal on-water surfski safety kit
Dawid Mocke on Safety
This is as much for would-be rescuers as for you. You stand a better chance of getting your lost gear back if you have identification printed on it. But you may also save someone a search mission to find you if they can get in contact with you when they discover any of your equipment.
You can also visually distinguish your gear with a tape color and/or pattern - particularly helpful when multiple paddler's gear is loaded in a trailer or piled on shore close together.
It is a good idea to carry a reliable visual signaling device particularly if you boat at night, and/or alone, and/or in waters that are challenging, remote, or have volatile water or weather conditions. Unfortunately flares are often or even usually kept too long to be reliable (Australia has a good program where they replace flares that have reached their expiration date - for free). Good electronic light signals may be getting to the point where they are as visible or better than flares, while also being more reliable.
Carrying a tow rope can help save equipment, paddlers and yourself. You can make tow rope with 20 feet of cord and two light carabiners. Clip the one carabiner to your rear handle and the other to the disabled ski's front handle or foot brace. If you attached it to the foot brace the paddler in the disabled ski will need to steer. If you do not have a rear handle you can add a secure attachment point using well nuts and pad eyes (carabiners are too large for small pad eyes so also add a cord loop or something larger to hook the biner to) or some other type of secure mount (all parts should be marine grade))