in deep water —(also: in deep waters) in serious (especially financial) difficulties:
- I have not suffered as you have. But I have been in rather deep water too, in another way.
in low water — = in deep water:
- Philip … came to know which of the clients must be treated with respect and which were in low water.
Note: The expression does not fully correlate in meaning with the phrase at low water—
1. when the tide is out and far from the shore:
- Filey Brig is a remarkable ridge of rocks, projecting half a mile into the sea and perfectly dry at low water.
2. (also: at low water-mark) = in low water:
- The money market was very tight; but in 1905 everything was even worse, everything was at low water.