Anything added to your deck should be low-profile and tidy - particularly if placed anywhere near your seat cockpit. You do not want to be catching your clothes or body parts on anything when you are falling out of or remounting your craft.
Many surfskis are poorly designed for quick and convenient drainage of water that gets inside the ski hull. One potential improvement is to adopt the bung hole/pressure relief tube style used in many outrigger canoes. Such bung hole plugs are inexpensive and readily available from outrigger companies. And drilling a hole at the rear end of you ski for such a plug is also quite easy. Use techniques recommended for drilling through fiberglass (tons of youtube videos for this) like putting painter's tape over the hole area, starting with an awl to make a small pilot hole, and using your drill in reverse mode (with a good hole bit - like a Forstner) to avoid fiber splintering.
Photo: Paul Reavley - Click to enlarge
In cold water when wearing bulky water shoes it can be difficult to get your feet back into the foot straps. This is especially true in big waves and when you have individual straps for each foot. By taping semi-rigid rubber inside the foot straps they will hold their shape so you can quickly get your feet into position and back to paddling.
A watch can be mounted on to the foot strap by strapping a piece of closed cell foam to the foot strap with zip ties. Then wrap the wrist band around the foam and the foot strap. The watch face is closer to see and to reach than mounting on the front of the footwell. GPS Deck Mount Examples
Technique can be the cause (bad technique) or cure (good technique) of a variety of discomforts in a surfski. But good technique does not solve all misfits between individual anatomy and surfski seating. Finding a surfski cockpit that fits your body is a good start and will help some paddlers more than others. Beyond that a paddler has to work on adjusting the shape of a cockpit to work as well as it can with the shape of their body while still striving to prevent those adjustments from interfering with good technique.
Padding is often needed in the seat area to adjust cockpits for better fit or to eliminate rubbing from hip rotation. If rubbing in the coccyx area is a problem, padding (with a non-absorbent material like closed-cell foam) with a channel in the center which will slightly raise each side of the buttocks leaving a channel for the coccyx (like bicycle seats) can be helpful. Likewise, rubbing in back at the base of the lower lumbar vertebrae can be alleviated by padding with a channel at the rear of your cockpit. You want to avoid friction and/or interference with proper hip rotation, so you may have to experiment with some combination of slick tape and paddling shorts (specifically designed to slide easily without seams to rub in the wrong places) to arrive at your ideal seat setup. Talking to other paddlers and checking out their solutions can be very helpful. But some paddlers apparently just grin and bear it, building up scar tissue in the afflicted area. If you use padding in the bottom of your seat it can possibly give you better leverage with your stroke, but be aware that it does not take very much elevation of your seat height to affect your balance and stability (read next paragraph)
Padding may also be used for balance and stability practice. Raising your seat height by a small amount can have an unexpectedly large effect on you sense of balance and stability. Conversely, some paddlers pad their seat bottoms to intentionally achieve this effect so they can practice with a tippier boat, improving their balance and experiencing greater stability when they remove such padding.
To avoid the sticky residue left by duct tape, use 1.5“ or greater black electric tape. No messy cleanup after removing your race number.
If there is space in the cockpit that you can fill with light, non-absorbant material like closed cell foam, you can reduce the amount of water that can fill your cockpit and effectively reduce speed loss when downwinding in big waves. The less water enters your cockpit, the less weight gain and difficulty getting back to speed while waiting for water to drain from your boat. Make sure that whatever you add is secure and does not interfere with water drainage or any aspect of your paddling, leg thrust, steering, remounts or boat handling.