Swim has a starting speed of 18KmH (walking speed starts at 25KmH, so it’s about 25% slower).
Each point in the swim skill improves your swim speed by 0,8% or 0,2KmH, each negative level below zero swim reduces speed by 0,1KmH or 0,4%.
There are several bonusses to raise the skill.
+78 – Strength modifier (from the 166 Strength)
+20 – Item enhancement bonus
+23 – Spent skill points
+5 – Skill boost
+6 – str skills (ioun stone)
+5 – feats
+2 -exceptional skill bonus
+4 – Greater Heroism
+8 – Human enhancements
+7 epic skill feats
That makes the highest swim speed 47,4Kmh, or more then double the starting swim speed.
The swim skill influences how long you can stay underwater, the formula for this is “19.5 + (swim level x 0.5) = amount of seconds”, for 164 swim this is 101.5 Seconds before you start getting damaged.
The damage seems to be 1.3HP per second or about 80HP per minute.
Currents are different per quest, some aren’t possible to swim against, but Crucible has a current of 18KmH, which one could swim upstream