A Matter of Britain
Due to Armor penalties, Swimming is not a terribly useful skill on its own—PCs would have to have a skill in the 20s to have any hope of making a roll while wearing armor! While this is realistic, it also begs the question of having a Swimming skill at all.
Thus, the Swimming skill works differently from other skills. The rating in Swimming is treated as a bonus to an unopposed DEX roll made whenever the scenario calls for a Swimming roll.