...i think one of @TuxedoDragon's comments on one of their pooltoy transformation arts describe it best:
"#this would be such a stim for me tbh
#can you imagine?? the sensation?? of being so light and bouncy??"
...the interest comes more from the sensory aspect, rather than just being soft, i'd wager.
...first, you're made of thin sheets of rubber, not much else. smooth, simple, clean, though perhaps still with seams to add variance. perhaps with some stretch, pulling and snapping back painlessly... perhaps another stim to enjoy.
second, instead of plush filling, you are full of air, with enough pressure to keep you firm in your shape, to whatever extent you desire, while still allowing you some squish... perhaps you could let the air leak out of you, and feel what it's like to flatten, and fold in on yourself... or the opposite, inflation fans would get a kick out of it to be sure...
...third, as tux notes, you are extremely light and bouncy, similar physics to a beach ball. this also allowing you to enjoy floating on the surface of water, pushing down only forcing you back up, not to mention the lack of lungs, meaning you can enjoy the waters without a single worry...
...both becoming a pooltoy and a plushie seem like fun, just in different directions of fun.