Four letters are printed in plain block capitals: B, X, M and E. Which one of them looks exactly the same as itself in a side-to-side mirror image (left and right swapped) but does NOT look the same in its water image (top and bottom swapped)?

Check both flips for every letter. M has a left half and a right half that match, so a side-to-side flip leaves it alone; but turned top to bottom it becomes a W shape, so its water image is different — M has exactly the two properties asked for. B and E have a top half and a bottom half that match, so their water images are unchanged while their mirror images are back-to-front: they fail the first test and pass the wrong one. X is the trap for anyone who reaches for the most symmetrical letter — it survives both flips, so it fails the 'does NOT' half of the question.