There are many accessories that will enhance your kayak and sea kayaking experience - both from your personal comfort level and also from a safety aspect. 

PFD with hydro-pack pocket
Power Blade Double Paddle
Banjo Paddle with aluminium or fibreglass shaft 
Spare break-down paddle
Paddle leash
Spray Deck
Dry bags
Protective clothing including hat, jacket, gloves, boots
Pump
Navigation Light
Trolley (take apart)

For Sea Kayaks:
Inflatable Paddle Float or Foam Paddle Float or Sea Kayakers Ladder for self rescue
Towline
Sail