Assuming that your water parameter is part of your music event, just specify a seek speed for that parameter. A parameter with a seek speed will automatically shift to each new specified value at a rate no greater than x units per second, where x is the value of the parameter’s seek speed.
For example, setting the seek speed of your water parameter to 0.5 means that it will take 2 seconds to go from 1 to 0 or vice versa; Setting the seek speed to 2 means that it will take only 500 ms, and setting it to 1 means it will take exactly one second.
To set a seek speed in FMOD Studio, click on the name of the parameter where it appears next to the parameter controls, then adjust the “Seek Speed” knob in the deck.
Note that seek speed only affects event instances that are currently playing. The parameters of stopped event instances change instantly despite seek speed, and the parameters of paused event instances do not update until the event is unpaused.