As far as I know, there are no rules for this, so, I am asking for some other opinions on what rules to adapt and apply to it, because I can easily foresee this being very important in the next session of the game I am running.
Two strong characters are on dry land holding a long rope tied to a third character. The third dives underwater. How quickly (how many feet per round) can the two pull the diver back up to the surface?
To further complicate this, what if the diver is not dead weight? What if they actively swim towards the pullers? Could you really just combine the two speeds or is it impossible to swim while getting dragged? What if you can't swim at all? Can you still help by pulling yourself along the rope you're tied to?
And how about a useful extrapolation not as immediately relevant to me, but still useful to know:
What if someone is at the bottom of a cliff tied to your rope. How quickly can somebody pull them up the cliff (could be relevant if there are pursuers)? Again, what if the person is actively climbing the rope you're pulling?