Swim Speed
Source Player Core pg. 412
With a swim Speed, you can propel yourself through liquids with little impediment. Instead of attempting Athletics checks to Swim, you automatically succeed and move up to your swim Speed instead of the listed distance. Moving up or down is still moving through difficult terrain.
You might still have to attempt checks to Swim in hazardous conditions or to cross turbulent water. You can also choose to roll an Athletics check to Swim rather than accept an automatic success in hopes of getting a critical success. Your swim Speed grants you a +4 circumstance bonus to Athletics checks to Swim.
Having a swim Speed doesn't necessarily mean you can breathe in water, so you might still have to hold your breath if you're underwater to avoid drowning.
You might still have to attempt checks to Swim in hazardous conditions or to cross turbulent water. You can also choose to roll an Athletics check to Swim rather than accept an automatic success in hopes of getting a critical success. Your swim Speed grants you a +4 circumstance bonus to Athletics checks to Swim.
Having a swim Speed doesn't necessarily mean you can breathe in water, so you might still have to hold your breath if you're underwater to avoid drowning.