Perform a stutter shortly before entering water, typically through a door transition, by briefly releasing and then repressing forward before, but as close to, the transition as possible while continuing the dash.
This carries the stutter into the water where Samus will run in place while building the shinecharge, Allowing Samus to enter the water with little momentum.
After entering the water, dash should be held the entire time. With Gravity Suit it is possible to do this without leaving water, with or without a room transition, by removing the suit just after stuttering.
The same technique is applicable with acid.
See also: Tech Page