Yesterday evening, me and a friend tried to swim over to the prison island. Swimming without ,,turbo,, works pretty good, it was just a little notchy, but when we tried to swim faster it always desyncs us back to the position we would have been, when swimming without turbo.
So for now it is not possible to swim fast, because of the desync. It looks like the desync is reducing the character to the exact speed of swimming slowly.