To back up what @Zachary said, it looks like WCUSA was purchased by a British company named Rank Organisation in 1990, and Busch Entertainment bought the park in 1992. According to the book Amusement Parks of Virginia, Maryland, & Delaware by Jim Futrell:
(Side note: if you're interested in super detailed history of the region's theme parks, Futrell's book is as good as they come.)
Busch Entertainment Corporation saw that Water Country USA would make the perfect complement to its theme park and in 1992 reached an agreement to acquire it from the Rank Organisation. This large British conglomerate had purchased the park in 1990 from another British company, Mecca Leisure, which in turn purchased it in 1988 from its original owners.
(Side note: if you're interested in super detailed history of the region's theme parks, Futrell's book is as good as they come.)