You are a fisherman and a scuba diver. Sit-on-top kayaks are far more suited to fishing than sit-inside kayaks, and you can anchor a sit-on-top kayak and dive from it. Sit-on-top kayaks are also safer than sit-inside kayaks, because if you end up in the water, it’s easy to climb back onto a sit-on-top kayak, but much harder to get back into a sit-inside kayak, even with the help of a couple of other kayakers.
But before you spend money on a sit-on-top kayak, I suggest that you determine if you can climb back onto it from the water. You may never flip over your kayak, but if you do, and you can’t climb back onto it, you could be in a lot of trouble. So try climbing back onto a sit-on-top kayak in a pool or next to a dock. Your scuba diving experience should help you. Go with an experienced kayaker or salesperson so they can help you with the technique of climbing back onto a kayak. If you can’t do it, I suggest that you forget about kayaking and get some other watercraft. To kayak safely, you need to be able to climb back onto a kayak from the water, and you need to practice that skill so you can do it quickly and easily if you get dumped out of your kayak.
Here’s how to climb back onto a sit-on-top kayak: Grab the gunwale with both hands. Kick your feet so you’re lying horizontal in the water. With one fast motion, pull the kayak underneath you, and then quickly push forward until your head is sticking over the other side of the kayak. Then roll over on your side and sit up. Practice until you can right your kayak and climb back onto it in about 20 seconds. I snorkel and speargun for pike in Montana from a sit-on-top kayak while wearing a drysuit, about 15 pounds of weight on my body, and about 3.5 pounds of weight on each ankle, and I climb back onto my kayak many times a day using this technique.
Floating vertical in the water and trying to chin yourself on the gunwale of a kayak won’t work; you’ll just pull the kayak over on top of you. I don’t find rope stirrups to be at all helpful, and you can spend a lot of time in the water deploying them, but some people use them. If you don’t have any gear in the rear cargo area of your kayak, you may be able to pull yourself up on the stern of the kayak and work your way forward. Because you are a big guy, it will be easier for you to climb back onto a large and wide kayak like a Hobie Pro Angler.
If you want to fish from a kayak, consider getting a pedal kayak, which will leave both of your hands free for fishing, eating lunch, putting on sunscreen, sending pictures of your fish to facebook, etc. Pedal kayaks tend to be more expensive than paddle kayaks, but they have some huge advantages for fishing. And you aren’t going to be able to buy much of a motorboat for the cost of a brand new pedal kayak.