I have never seen a disabled person use one of those slings. Transferring into it from ta wheelchair would be dangerous and terrifying.

Why not build either a semi-inground pool (much cheaper), or make a dry descending ramp, and leave a wall at the seat height, so the user can make a direct transfer to it just like getting into and out of bed? Just transfer to the wall, and pick up the legs and swing them over. About a foot wide, could even be upholstered with padding and vinyl, or closed cell foam. I designed a spa like that for a wheelchair user, and it was a piece of cake--she just brought a folded towel with her to put on her seat cushion to use when she got out to keep it dry. With the buoyancy and normal chair user arm strength, it was no problem to get back out from a normal height bench inside the spa. For someone less strong, it could be stepped up along the wall, to enable scooting from level to level, or even ramped.