//Swim toggle
alias +swim1 "+moveup"
alias +swim2 "+movedown"
alias -swim1 "-moveup"
alias -swim2 "-movedown"
//Change the keys here if your controls are different
alias swimmode "bind space +swim1; bind shift +swim2; echo "Swimming mode enabled""
alias "smToggle" "sm1"
alias "sm1" "swimmode; alias smToggle sm2"
alias "sm2" "bind space +jump; bind shift +duck; alias smToggle sm1; echo "Swimming mode disabled""
//Change this key if you want to press a different key for swimming
bind b smToggle

//Optional code to reset controls upon class switch
bind space "+jump"
bind shift "+duck"
alias smToggle sm1

This script allows you to swap from land controls (crouch and duck) to water controls (swim up & swim down) at the push of a button. Pressing B (by default) activates the code. It's a bit sloppy, perhaps, but it gets the job done.