I think John's problem here is getting the Sea Dek off. I used to own John's boat and installed the Sea Dek about 10 years ago. So here's my .02.
John, your going to have to get the sea dek off completely and the deck underneath reasonably smooth, no matter what you go with as a replacement. No easy way to do it, but it shouldn't be super adhered, I peeled and stuck it using the pre applied adhesive, no epoxy or anything. I think I'd try scraping it, maybe use some heat. After I got as much off as possible, I'd go back and grind the rest off with a high grit sander. That might be a job to consider hiring out, it's going to be fairly labor intensive. The boat had another type of mat/non skid on the decks when I bought it in 2005, it eventually fell apart and I scraped/ground it all off. It was a big job. I was 40 then but I still was pretty beat down at the end of it. It's not skilled labor, just hard on the back. Talk to Brett Shields, he may know somebody that will do it reasonably or possibly the Lynn Brothers.
Aesthetically, I'm sorry to tell you that you're not going to like what is under the Sea Dek. The top skin wasn't leaking when you bought the boat, but it looks really bad. To get a good finish on the decks you would need to fair and sand it. There is a lot of ugly gel coat under the Sea Dek. That's why I decided to put the Sea Dek back on rather than going with a full deck grind/fair/grind/prime/paint. Given the current age and overall condition of the boat, I recommend doing something similar. Heck, the Sea Dek lasted 10 years, that's not so bad. If you recall, it looks really nice and feels nice underfoot when it's done.
I may be wrong, but I think Kiwi Grip wouldn't be great in your situation without a lot of prep. I've heard it's a good product though.
The good news is, that you don't need to worry too much about damaging the gel coat when you grind the Sea Dek off. It already looks rough, and I can tell you from experience that the top skin is super thick on your boat. I repaired/recored a few wet spots in the deck and was always amazed at how heavy they laid on the glass in 1973.
Don's Plasti Teak may work as well.
I hope this helps and I hope that you are feeling better.
Doug