From the Ministry of Economy, Trade and Industries (METI)'s press release (in Japanese) on May 28 on the Fukushima I plant conditions, "Water levels at trenches":
Reactor 2 trench water level:
5/7: 880 millimeters (34.65 inches) from the top of grating of the trench
5/14: 760 millimeters (29.92 inches)
5/21: 760 millimeters (29.92 inches)
5/28: 576 millimeters (22.68 inches)
Reactor 3 trench water level:
5/7: 820 millimeters (32.28 inches) from the top of grating of the trench
5/14: 720 millimeters (28.35 inches)
5/21: 640 millimeters (25.20 inches)
5/28: 431 millimeters (16.97 inches)
The top edge of the trenches is at the Onahama Peil (O.P., average sea level used for the plant) plus 4,000 millimeters. In other words, 4 meters (13.12 feet) above sea level.