Is this true exactly? There's a bridge near me that is only 150ft tall, so the water clearance is going to be even less than that, but mortality rate for jumpers is 98%. Yes they may drown but regardless survival is very rare and probably due to jumping near the edge of the bank rather than centre of the river.
source:
https://en.wikipedia.org/wiki/Humber_Bridge "More than 200 incidents of people jumping or falling from the bridge have taken place since it was opened in 1981; only five have survived."
Edit: I suppose at lesser heights like these it matters a lot about the condition of the waters you're jumping into, too. The humber estuary has strong currents going out to sea I believe.