Yea the beaches are beautiful, but the water is way to cold to actually go in, except maybe in late Aug/early Sep.
Nightlife seems just ok. Yea you have the Gaslamp area, but it just seems fake to me. To get to real clubs, you have to either take an expensive cab or have a car.

I was just in Seattle for work a few weeks ago. I was stationed nearby while in the Army back in the early 90s and have been about every 2-3 years since. There's always something exciting to do. Heck, if nothing else you can go out on a ferry then come back, and it is pretty cheap.
Plus the light rail makes getting from/to SEA a breeze (and cheap).
In a month it could still be rainy, but just plan for the worst, hope for the best
Plus the club scene in Seattle is probably one of the best in the US, IMO.