If you are OK with a more permanent bond I use PL construction adhesive. It holds very good and the parts can be separated later if needed with a putty knife. Many adhesives dissolve different types of foam so a small test is always a good idea.

I agree with the idea of gluing the foam into the boat.
Something like construction adhesive (tested as suggested) would be ideal for a seat which needs to be adjusted into the correct position. For foam thigh padding and hip pads, I use solvent-type contact cement.

Just 'jamming' the seat in place with foam blocks looks secure, but I don't think it's a good idea. It's just too easy to accidentally dislodge one of those blocks - during a rescue (practice, hopefully) or if a friend is helping to move the boat, etc. .

Off topic:Also, jamming a block under the deck aft of the cockpit requires the bulkhead to be 'in the wrong place' in my opinion. Aft bulkheads should be as flush as possible with the coaming back, and sloped, to allow water to be easily dumped out of the boat. (see the NDK Romany-type boats). Bjorn Thomasson suggests integrating a hard seat with the bulkhead, which is another elegant approach if the seat position allows it.

My experience with 'industrial strength adhesive Velcro' is the same as mentioned - no matter what I did to prep the hull surface, it peeled off after a few weeks or months of salt dampness. If you need the seat position to be adjustable (fore and aft?) or removable and must use Velcro, the fabric type (without adhesive backing) can be carefully epoxied to the hull.
Removing it requires a grinder.