Cylinders and BCD's are usually strapped into the well on the back of the kayak but all your other bits like fins, mask, instruments, food, water etc. will have to go elsewhere. Some kayaks have seal able hatches in the bow which are great for stowing gear and some just have a well or deck space with bungee or webbing straps. Either way, gear is best stored in containers to keep it organised and protected. Simple, tough and watertight. They're great for storing just about anything that will fit in them. These are available in a range of sizes from 0.5 ltr to a huge 117 ltr. These will fit through most hatches or can be secured to the deck.
Lots of different shapes, sizes and colors. These are good for keys, phones, cameras, first aid kits or even food. Protect your personal possessions from the elements with a sturdy waterproof case.