Your title is the question, "What makes a Sea Kayak". The best answer to that is to define the needs of a true sea kayak and let you see if yours fits the bill. A true sea kayak needs to be able to fit the needs if the sea. That may include wind, waves, long distances between shore points and the need to be self suficient. Hence sea kayaks tend to be long (to travel the longer distances with some speed), and narrow (for both speed and boat control). Have small cockpits with securely fitting spray skirts (that won't take on water if a wave dumps on them)and provide a good gripping surface, under the deck, for you knees in case of needed boat control, including a roll. They also tend to be low to the water to cheat the affects of the wind, and designed to some degree to handle rough water. I would take this definition and see how your boat stacks up. Now, saying all of that, your boat still may be capable of day trips on the big waters, but you should be careful to pick the right conditions. AL