WATERLEVEL.BAT generates custom WATER.DAT and WATER1.DAT files to change the water level and wave size. Put it in your "GTA San Andreas\data" directory, make sure you backup your original WATER.DAT and WATER1.DAT files first, and run it with 1 or 2 parameters:

WATERLEVEL [waterlevelinmeters]
or
WATERLEVEL [waterlevelinmeters] [wavesize]

Examples:
WATERLEVEL 10.0 -- 1ft water in Las Venturas
WATERLEVEL 13.0 -- 2ft water in Grove Street
WATERLEVEL 13.0 1.0 -- 2ft in Grove St and ocean sized waves
WATERLEVEL 0.0 4.0 -- huge waves