Acrobatics to balance on top of the rope connecting the two boats, or Athletics to climb along it.
If you've got a 15 in Strength or Dexterity, I'll let you bypass the skill check, for simplicity's sake and to move things along. I'll probably apply that principle to more skill checks in the future to avoid the situation we just had of people being stuck in the water when they should have easily been able to get out, given their abilities and training.