Undo implications and alias to on_* counterparts. Biggest argument for keeping sitting_on_* is for example sitting on_desk could bring up someone sitting in a chair with another lying on_desk. However, the largest of these, ~sitting_on_desk ~on_desk sitting lying has only 22 active posts that contain both, thus currently would not result in many false positives for one of sitting/lying searches. It does make sense to keep standing_on_* tags as standing on these is an unusual occurence not easily found with a "standing on_*" search.
It feels a little strange that sitting on a puddle would be grouped together with standing in waist-deep water. For example, post #4254704 has the appearance of sitting on ice... with ripples and bubbles. Somewhat, post #5880827. Think of standing on liquid, but sitting. Do you really call these "partially submerged"? Understandably submerged if the water is a few inches deep and you can see the submersion, but I think we can keep sitting on water for "surface-like sitting on water" that isn't a frozen lake or merely wet concrete, and alias to sitting_on_liquid in case ambiguous red liquid shows up in future or something. Floating tends to involve an inner tube or partial submersion like realistic floating, so this isn't it.
Leaning back can be any standing/sitting/reclining. Reclining can be argued to be sitting leaning back, but I'd omit leaning back if it the back is supported without arm support i.e. actually relaxed as if lying on an incline. I'm not even sure if the arm support type is supposed to apply to reclining.