This clip is a great example of how not to pack your kayak. Notice the huge (and heavy) container of water packed on top of the boat? Sitting that high out of the water, it is causing his boat to become very unstable.
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Many of the sea kayak courses that I teach over the year involve going out for multi day trips. That has given me a chance to see a huge variety of different methods of packing a boat. Everybody seems to have their own method and that make sense since each boat is a little different in regards to space and everybody has different amounts and different types of gear.
If you are planning to do your first big trip soon and wondering how you are going to fit your mountain of gear into that little boat, don't worry. With careful planing and packing, you will (hopefully) get it in. My little rule of thumb is that an average 17 foot sea kayak will have about 1.5 to 2 times more storage then a large multiday backpack.
Learning to pack and stuff it into your boat is a learned skill. Take the time to do it right and your trip will go smoother and you will have room to pack even more stuff!
A couple of quick tips that helped me get started:
Figure out what needs to be waterproof and what can get wet. Large waterproof bags are great but take up a lot of space. Clothing and sleeping bags need to stay dry but cans don't.
If it goes in a bag, keep the bag small. Multiple bags are better then one giant one. Remember, the goal is to fill in every small amount of space in your boat. One large bag will waste a lot of space
Pack lots of loose items and fill in all your storage cracks. Fill in all the spaces between your gear with small items like fuel cells or fruit. When unpacking at the campsite, pull out a cotton bag or an Ikea blue shopping bag. They pack down really small and help with carrying those loose items.
Get the smallest gear you can get. Shop for smaller tents and sleeping bags that will pack down small.
Split up your tent with your tent partner. Pull it out of that giant tent bag. The fly and poles goes in one boat and tent body goes in the other. It's much more efficient of space.
Put your heaviest stuff at the bottom. Natural ballast. You can see what happens in the video above if you put the weight up top!
Squeeze as much air out of your gear. I have seen this so many times. Air in bags equil lost space. Take the time to get the air out and you might just end up having room to bring that apple pie you were eyeing at the grocery store!
Of course, that just scratches the surface. There are a 1001 tips and ideas to help you out. Here are a couple online articles to get you started:
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