I'm pretty much the country's foremost expert on lounging at hotel pools (especially while not a registered guest). As a matter of fact, I'm lounging at one now awaiting my XXX-ORD-MIA flights enjoying complimentary WiFi & coffee.
Most hotels only allow registered guests in the water facilities (insurance reasons). Exceptions include full-scale properties with a day-spa.
As far as I know, there is no pool-only rate.
Here's my IAH expertise:
-Marriott (on airfield, accessible from the Tram) As of July 2015, the entire property, including the outdoor pool is undergoing renovations. I'm sure it will be key card access only. Their nightly rate is a bit steep (I'm sure the day rate is as well).
-Sheraton decent indoor pool and hottub without card key access
-Hampton JFK Blvd small outdoor pool with card key access
-Staybridge very nice outdoor pool with card key access
-Doubletree nice outdoor pool, open access
**if you're top-tier Elite at IHG or Hilton, I've had an extremely high success rate just approaching the front desk, state you're on a lengthy connection and they usually store your bags and issue you a property access key. This is, of course, totally discretionary. Tips (in the form of gifts work).
YMMV
Hope this helps!