Some creatures have aquatic speed, but playable classes do not.
Does this imply that PCs cannot swim? Or does swimming count as difficult terrain? Or do they just have the same speed? Or is an athletics check necessary for water movement?
Yes, PCs can swim.
Special Types of Movement
Movement through dangerous dungeons or wilderness areas often involves more than simply walking. Adventurers might have to climb, crawl, swim, or jump to get where they need to go.
Climbing, Swimming, and Crawling
While climbing or swimming, each foot of movement costs 1 extra foot (2 extra feet in difficult terrain), unless a creature has a climbing or swimming speed. At the GM’s option, climbing a slippery vertical surface or one with few handholds requires a successful Strength (Athletics) check. Similarly, gaining any distance in rough water might require a successful Strength (Athletics) check.