The speed at which a character can swim depends on the style in which he/she is swimming and how good he/she is at swimming. Every five seconds, a character should make a dice roll using his/her CO + Swimming (including Swimming speed specialism), subject to a modifier to the basic value for the style of swimming adopted. The number of metres that he/she moves in that five seconds is then equal to half the dice roll result.
For example, Roger, an expert swimmer with a Swimming score of 5 (but no points of specialism in Swimming speed), has a basic CO of +1/2. When unencumbered (+2 modifier to his CO) and doing the crawl (+2 modifier, see above), his basic value for Swimming speed is 5 + 0 + 1/2 +2 +2 = 9 1/2 (which means he can get a certain dice roll result of 9). So, when he does the crawl, he can typically travel approximately 4.5 metres in 5 seconds, i.e. at roughly 3.25 kilometres per hour.
When lifesaving his friend Charles (see example under encumbrance and life-saving) and dragging him to shore using the backstroke, Roger's basic value for Swimming speed is 5 + 0 +1/2 - 5 1/2 +1 = 1 (which means he can get a certain dice roll result of 1), so he typically travels approximately 0.5 metres in 5 seconds, i.e. at roughly 0.375 kilometres per hour.