"Dwarves can't swim" where did this come from?

[Quizatzhaderac]

I would imagine that living underground, there is a dearth of oceans/rivers/lakes/swimming pools in which to learn the skill.

Water flows downward, so it's more the exception to have caves filled with air than water.

Maybe one of the favorite pastimes of dwarfs is to relax at the under-beach, soaking up the dark-sun, with a nice room-temperature koruna™ beer? Maybe enjoy a game of volley-axe, or do some shadow-surfing.

I thought it was from Dwarf Fortress. Something to do with undead carps or something?

Reference is definitely too new for that. Also, DF dwarfs can swim (even untrained), it's just a matter of finding water without high CR monsters in it (like carp).