It's easy to turn sideways in a sit-on-top kayak to reach gear in the rear storage area, and you will be stable and comfortable when turned sideways. I do it many times a day.
It's more of a hassle to slide forwards to reach gear under your front hatch, because you need to reach over your Mirage Drive. When you get into the area under your front hatch, hang on to the hatch cover. If you throw the hatch cover all of the way open, you'll need to crawl way forwards to grab it so you can close it.
On a prior kayak, I mounted pockets for gear on the back of my seat. See
http://www.northwestkayakanglers.com/in ... #msg128575 . It was awkward and inconvenient to reach back to get gear when I was facing forward in my seat, particularly since I couldn't see what I was reaching for. I ended up turning sideways in my seat to get at the gear on the back of my seat. So while seat-back gear storage can be useful, don't count on being able to get at the gear without turning sideways in your seat.
On my Revolution, I store pliers, a fish bonker, a folding knife, shears, a lip gripper, and attractant bottles in the mesh pockets next to my seat. This is way more convenient than storing them on the back of my seat. But I still turn sideways to get at gear in my rear storage area.