2 floodings for a 3 upkeep flooding with the same power as now? I don't like it.
Problem with "stackable" flooding is reducing the field to 0 is just way too strong in my opinion.
Maybe something else (each "flood" increases maintenance by 1):
1 flood - 5 tiles left
2 flood - 4 tiles left
3 flood - 4 tiles left
4, 5, 6, 7 flood - 3 tiles left
8 flood 2 tiles left
(well just showing 8 to show the calculation: to remove an extra tile, you would need to double the cost).