There are two ways to get into your boat when you're in open water, not by the shore. You can climb into your boat via the back, or you climb into the capsized boat and roll yourself up.
To begin with, wade into chest deep water with your boat next to you.
Wear some comfy, but robust clothes to avoid scrapes and bruises.
The simple way to do a wet entry is when the boat is upright. Practice this first until you really confident.
Before you slide into the cockpit let some water run out of your clothes for a moment while you sit on the back of the boat, or you flood the boat. Especially waterproofs can hold a fair bit of water in the sleeves and pants. Raising your arms usually drains the sleeves.
With this wet entry, you enter the kayak upside down, and then roll back up. Due to the water supporting your body weight these re-entry options take a lot less energy than climbing back on top of your kayak. The downside is you will scoop up quite a bit of water as you roll up.
Being comfortable when upside down in your kayak is essential when learning how to roll. It is also be a necessary component of the following skills:
Let's go through the rescue step by step. After the wet exit, surface next to your kayak being careful to hang onto all your equipment. Face the bow of the upside down kayak.
Visualise which leg should go where in the upside down boat. This is very important since it can get very confusing figuring this out once you are upside down.
Put the paddle along side the boat and up against the coaming. This way you can hold onto the paddle but still have both hands available to help with the re-entry.
Then take a deep breath and rotate sideways to submerse yourself. As you go under, push your legs into the boat and pull your butt into the seat.
Position your hands on the paddle for a roll sweeping out to the side from which you entered the boat, brace your legs for a roll (preferably on the foot pegs). Sweep the blade with the float away from the boat and hip flick to roll up.
If you have good lungs and a not-too-tight spray skirt, it helps greatly to re-attach the spray skirt before you roll up.
You may be able to get halfway into your kayak while it's still on its side, so you can breathe and take your time
It is also feasible to get into the boat by surfacing inside the overturned boat, head facing the stern of the boat. Grab the coaming with both hands, holding the paddle also in one. Now rotate (somersault) the legs between your arms and insert them into the boat. Pull the boat on, like a pair of pants, get a grip with your thighs and roll up.
As an alternative, try practising with the paddle first put under the deck bungies.
That allows you to ignore the paddle until you're in the seat.
This is only viable if your kayak is rigged to hold a paddle securely.