How to /testfor player in water
/execute's detect mode will allow you to execute a command from an entity only if they succeed in finding a block. For example:
/execute @a ~ ~ ~ detect ~ ~ ~ water * /effect @a[c=1,r=0] speed 1 11
If you only want it to work for PlaycastSteffen, then you can do:
/execute PlaycastSteffen ~ ~ ~ detect ~ ~ ~ water * /effect PlaycastSteffen speed 1 11