Finally found USAF HU-16B manual (T.O. 1U-16(H)B-1), which is more conservative, says on pg. 2-25:
"When selecting an area on a river, lake, or bay for operation, the minimum usable area should not be less than 5000 feet long and 300 feet wide. If obstructions exist, takeoffs and landings should not be attempted prior to computing climbout/landing data. While an aircraft weighing 27000 pounds or less can operate from this area, a larger area will be needed for weights over 27000 lbs."
However, there are no landing distance charts for water landings. They use a 500' pattern altitude for normal water operations. But what Wacopilot says for landing squares with the hard-surface runway landing distances, can always splashdown shorter than on dry land from touchdown. They're (USAF) probably counting you want to get back of the water, eventually, with engine-out performance.