While this is a reply to Rob, it is more addressed at people with less experience than Rob. He knows enough to make informed choices.

My copy of Derek Hutchinson's "Sea Canoeing" that I purchased in 1986 talks about deck lines, and imported British kayaks really started the sea kayaking movement in the US in the late 1970's, early 80's, and of course Greenland Inuit kayaks almost always had some deck lines, and they were common on Alaskan kayaks, so it is not exactly an upstart idea.

The consequences of being separated from your boat in sea kayaking are typically much more severe than whitewater. Unless you are on big rivers out west, in whitewater you and your boat will likely fetch on land within a few minutes with minimal effort from the swimmer. Where on the sea, your next natural landfall may be another continent. Even a 10 knot wind can permanently separate a swimmer from his sea kayak. A system that depends on the swimmer acting at 100% competency is doomed to failure.

The advice to never let go of your paddle and your cockpit is excellent and fairly standard for sea kayaking, yet s#!t has been known to happen. Deck lines are one more line of defense against the worst happening. They make a slippery object easier to grab in the nasty conditions that are most liable to cause a capsize.

Holding on to the cockpit can also be very difficult in surf and breaking waves. It is sometimes best to move yourself to the bow or stern so the boat slices through the waves instead of being carried along by the waves. Accomplishing this without a series of functional hand-holds between the cockpit and the ends is risky.

Almost all the self rescues and assisted rescues that are viable in rough conditions can successfully be accomplished without ever grabbing deck lines. Then BCU coaches I have worked with model the skills without using deck lines. They usually are most reliably done when you don't have to rely on deck lines, but there are few situations where you would regret having them. I am a strong believer in belts and suspenders. More than one means of keeping your pants on is good practice if you life depends on pants.

I say all of this as someone who does not always install full deck lines on all the boats I make for customers. It is a choice I leave up to them. Many people like wooden kayak for the aesthetics and prefer the aesthetic of a clean deck, but I do suggest deck lines, bulkheads, and float bags as being the safest options. I try to inform them about safety, but that is not always their top concern. I've made more than one kayak without any of the above because that is what the customer wanted. I like to place some form of deck line within an arms length of any part of the kayak, but sometimes the customer doesn't want anything competing with the wood. People are free to make choices. Hopefully I have made the choices clear to them.