John -
I fish several days a week in the Severn from my Native Manta Ray 14 (sit on top style). I am a similar age to you and have similar stiff and sore body issues too. Ibuprofen keeps me going on the water. The Native Ultimate is also a nice kayak, but is laid out part like a canoe and part like a kayak. It has a very comfortable seat according to most people (however, I paddled one for a day trip with a kayak fishing guide and did not like the way the seat bars poked into my back). The seat on my Manta Ray is extremely comfortable for me and feels different from the Ultimate seat.
One advantage of a sit on top kayak is that they have scupper holes in the floor. If you take a wave over the hull into the cockpit it will drain out the scuppers. Also, it is not uncommon to get a small amount of drippage on each paddle stroke. If you do not have any means for that water to escape, it does accumulate after a while. The Ultimate does not have any scupper holes, so you would need to carry a sponge or a pump to get water out.
If you want to see the Ultimate, there are two Native dealers in the greater Annapolis area: Annapolis Canoe and Kayak in Eastport and Kent Island Kayaks near Kent Narrows.
You are welcome to look at and sit in my two sit on top kayaks to get a sense of how they feel. But there is no substitute for trying out a kayak for an hour or so to see how it performs under real on-water conditions. If you buy a kayak and it is not comfortable, you will end up not using it much.
Although I personally am a fan of paddle style kayaks, many kayak anglers prefer the Hobie kayaks that use a foot pedal propulsion system. If you have serious back issues that might limit your paddling ability, consider a Hobie model. There are some models of the Ultimate that can be fitted with a Native foot pedal system too.
John Veil
Scout 162 Sportfish, Native Watercraft Manta 14, and Ocean Kayak Drifter