Yeah, no doubt about that. But, the question remains is it needed to have much higher than about 1.01 density since that makes you weightless? More solution density just means you float higher. I would think that for the sensation of weightlessness and relaxation, all you need is enough bouyancy to keep your head above water and that can be helped with a scrap of polystyrene behind your head. Seawater should just about do the job and its only about 1.03 gm/cc.
Would you feel more relaxed with a super high density liquid? What about mercury? You would hardly sink into it with a density of 13.53 gm/cc. I think you would feel like you are laying on a soft surface rather than floating. I think floating in a solution that is not so dense would feel better and since the highest possible density is not the best, what would be ideal? I'm thinking just dense enough so that you don't have to be concerned about holding your head up. If you are going to float vertically, as I was speculating about, the float for your head might be the best way, otherwise you might have to stay with horizontal floating. You could test it by using a float for your head and going to the ocean, if you don't live too far away.
I always found it very relaxing to snorkle and just float in the water. A life jacket is another way to do it but then you have the sensation of wearing something. It would let you get an idea without spending $50 for an hour in a spa or thousands for a special tub in your home. If you have received shipments with those plastic air pouches for packing material, use them. I think I would try that first, if I like it well enough, I might go to the spa and see if its worth the extra money.
Those water tanks are not cheap either. It would cost around $400 for one big enough but it would not take as much space as a tub. You would need ladders to get in and out. Use syphon hoses to drain it and bring in a hose from a faucet to fill it. It would take around 400 gallons to fill it so the epsom would not be super cheap, depending on what density you wanted. I think 50 lb would make you bouyant enough that your head would come up, still might need a floatation device to be comfortable.
My next project is to go to the beach on a quiet day with some sort of float and see how nice it is. You would not have total isolation but you could experience the floating and see how it is.