Hi,
I have tried several hotels that I have stayed at in the past ( ie Hilton Dulles airport, Marriott key bridge) but they say the pools are closed
However, the Marriott Crystal City @ Regan National airport site suggests their pool is open ( ie M-TH 2pm to 9pm and Fri-sun 10am to 10pm) but reservations are needed and capacity is limited. I would still call to confirm as policies could change
Details are here ( under property amenities)
https://whattoexpect.marriott.com/wascc
Regards
TBS