Disney World saw "lower results" on attendance and revenue, while SeaWorld reported a nearly 3% attendance drop across its parks.
A SeaWorld statement said, "Unfortunately, we do not see a path to a transaction."
Legoland Florida will scale back distancing markers to three feet starting May 14.
On the plus side, per capita revenue, admissions, and in-park spending increased in 2020.
Aquatica San Diego will instead have one final operating season in 2021.
Comcast CEO Brian Roberts said some guests may not return to theme parks until a COVID-19 vaccine is available.
"SeaWorld has determined that it must transition certain park and corporate personnel from a furloughed status to a permanent layoff," the company said in a statement.
Also delayed will be Emperor, Pantheon and Ice Breaker.
Busch Gardens Williamsburg is one of three SeaWorld-owned parks that remain closed.