best way to level up swimming?

[WarpathEQ]

It takes nearly a full day of constant movement in water to max out swimming, the skill ups come very slow at the higher levels but its worth it, max swimming is like having SoW on underwater.

My favorite place as a bard is in Cobalt Scar, there is a rock that sticks out of the zonewall between SG entrance and the Beach that has a downward slope. You can put on selo's song of travel, set your view to facing max up, and set auto run to on. The downward slope will ensure your legs are always in the water and it was much faster skill ups than anything else I could find without any user inputs needed.

Other pro tips:
Anytime you're in a zone with water you can run along the shore, if your feet are in the water, even if you're still running and not swimming you will get skill ups. I "trace" the shore in zones like lake of ill omen and innothul swamp to add a couple of skill ups each time I run through.

Running on the ice river in Everfrost skills up your swimming so another good spot you could auto run into a wall and get skill ups. There is also a pavilion in Lake of Ill Omen in the lake that has a floor you can run on but is in the water, you can auto run directly into one of the 4 pillars.

There seems to be a component of distance (or potential distance) covered along with presumably an RNG check to give a skill up. So I find the faster you are moving (i.e. sow speed+) the faster the skill ups will come, that's why I recommend a strategy that involves running skill ups as opposed to being fully submerged.

EDIT: to be clear, you don't need to be actually moving (changing location) to skill up. Auto running into a wall/object without location change does result in skill ups.