Does it have to be an outdoor pool specifically? Not too many hotels IN DC have outdoor pools. More have indoor pools if you can compromise with that.
Trip Advisor is a good resource too. It will rank hotels by reviews. They have pretty accurate reviews and lots of traveller photos.
The hotels that we have stayed at that have outdoor pools in DC are: L'Enfant Plaza hotel (ugh, would not recommend) and across the river the Key Bridge Marriott in Arlington (very nice hotel). We've stayed at the JW Marriott in DC which has an indoor pool. (great location, ok hotel, pricey not as nice as other JW's)
We've not stayed here, but visited and the Omni Shoreham has an outdoor pool which looks nice.
We've stayed at lots of other nice hotels but did not have a pool, and I know it is nice to have one for the kids after long, hot days.
PS Also, some hotels have reciprocal agreements with others to let you use their pool~ something to check into.