Put a 3/4 plywood 'deck' over that tub, wall to wall, before you start taking down the tile. Nothing worse than chipping that tub glaze. Also will make clean up much easier. As for how much material you 'need' to strip, (call me Mr. Overkill![:I]) But I agree with taking it down to the studs. Only then can you really be sure of how far the water damage is, plus, it may provide an opportunity to insulate those walls and put up new board. The cost of that backboard is cheap compared to missing a problem spot. And, that new board will be a nice, smooth surface ready for that neat NEW tile your wife will be wanting!
