I'm only seeing a condition function for http://www.creationkit.com/IsSwimming, I don't see a Papyrus version. If you need this in code, then the only thing I see for swimming is an animation variable, so I'd take a look at this: http://www.creationkit.com/List_of_Animation_Variables.
You could try this code:
Debug.Notification("Swimstate="+GetAnimationVariableFloat("SwimDirection"))
Put this in code where ever it is you have to check and see what it shows anything useful. If it changes state when you're swimming, then great, your IF/THEN would look something like:
If GetAnimationVariableFloat("SwimDirection")) > 0 SetAV("SpeedMult",160)Else SetAV("SpeedMULT",130)EndIf
Note: I don't know if it's a float, might need replace GetAnimationVariableFloat with:
GetAnimationVariableInt
GetAnimationVariableBool <- probably NOT a bool, but if the other two don't work, might as well try.
I'll let you do the actual testing so you can save your sweet rolls
Edit: BTW, did you do a search here on the forums already? It would be surprising if you are the first to ask about swimming.