Howdy and welcome up here (from an ex-Houstonian...)!
You'll find water temperatures up here in the balmy low 50 range, probably 52 - 54 at the highest, depending on when and where you're going to be diving. You can do it in a 7mm farmer john, but you'll be happier diving dry.
Give us a little better idea of where in the Seattle you'll be, as this area's littered with dive shops, good, bad, and otherwise, and we can probably make better recommendations for you. Also, it'd be helpful to tell us what sorts of diving you'd like to do...are you looking to do some shore diving (there are fabulous sites all over Western Washington) or would you prefer a boat charter? Do you need anything in particular (rebreather support - divesorb/100% O2? Mixed gas?) Is there something in particular you want to see?
The Puget Sound is the easiest place to access from Seattle, and there's lots of great parks (including an underwater one in Edmonds...) where diving's frequent. The Hood Canal out to the west is a little less current intensive and a little less rich in life. The San Juans are the crown jewel of Washington diving, but they can get rough if you're not used to high currents and low visibilities. Most of the good San Juan sites are boat-only, but boats are pretty easy to find out of Friday Harbor or Anacortes.
Give us a little more info, and we can make much better recommendations for you.