Like other aspects of gear selection, choosing the proper seat, pads or backrest should be based upon the premise that the user has proper paddling posture. If you are considering a seat or backrest because you get sore or tired after every paddle, perhaps the reason is that you aren’t sitting correctly in your cockpit, thereby putting excessive strain on certain parts of your body (lower back, rear end, even your feet and legs).
Kayak cockpits have come a long way in providing comfortable shapes and sizes in seats and in a variety of adjustments that can fine tune the “fit” of your boat. However, if you still need to make personal comfort adjustments, there are a variety of padded accessories to help you hone your comfort level.
As with most things in life, when it comes to kayak seats, you've got some options. Kayak seat cushions can reduce, spread out or eliminate pressure points along your posterior. Adjustable backrests can offer support to your lumbar region and both can provide a bit of insulation against the hardened cold of an uncushioned seat. Various cushioning materials are used to pad seats/backrests and most manufactures provide straps and hardware to help secure the units into position.
In addition to seats and backs, consider pads for your knees (bracing in kayaks, kneeling position in canoes) as well as foot/heel pads for operating the rudder or merely as a chafing guard for your heel when pressing against a kayak’s foot brace.
However the pads are secured - adhesives for floor/knee brace mountings or straps and non-skid surfaces for seats, it’s important that they don’t constrict your motion nor prevent you from proper bracing and support positioning while going through different paddling motions and boat maneuvers.
Some seat backs rise above the level of the aft deck, providing more upper back/leaning support. However, a raised backrest can also restrict certain rolling procedures by preventing the paddler from leaning back far enough on the deck to execute the roll efficiently - or at all.
Some seat systems provide side pouches and accessory stowage options along either side of the cockpit or behind the backrest. As a safety issue, make sure there are no catches or other attachments on the seat that could restrict you from exiting your boat in an emergency. This is also a concern regarding items stuffed or stowed in the cockpit area that might come lose or otherwise inhibit a quick and safe exit during a capsize.