Swimming

The swimming ability is the gift of Rahab and his clan's adaptation to water. While vampires aren't supposed to be able to be in sunlight, it seems that after a lengthy exposure to weakened daylight that they'll adapt and be able to stand it. Such is Kain's empire with the sun muted by the smoke of furnaces so that vampires can eventually adapt to sunlight. Perhaps he got this idea from being in Termogent Forest (or Black Forest as called by Ariel in Defiance). When Raziel visited the forest he did say that light barely shone through the dense forest. In the prolouge of Soul Reaver 1 Rahab can be seen in the sunlight. It seems that over time he and his clan lost this adaptation to replace it with adaptation to water and thus they learned and mastered swimming so much that they are becoming more water creature-like. Perhaps the reason why lies with Kain's mist ability/form.