In August, the water will likely be in the mid 50's, and people do dive it in 7 mm wet suits. Visibility varies -- At that time of year, you can have fairly poor viz if the cycle of algae blooms is at the crest, or you can have windows of excellent viz during the die-offs. Puget Sound is pretty protected water, so you don't have to deal with surf entries. We have a lot of easily accessible shore diving, but a lot depends on where in the Seattle area you will be staying, and what the exact dates of your trip are. Some of our best sites are current-sensitive and must be dived on slack, or on days with small tidal exchanges.
Pretty standard sites, and diveable in most conditions, are the Coves at Alki and the underwater park at Edmonds. I like to take visitors and people new to me to the Alki Pipeline, which has more life than the Coves and is also not very current-sensitive.
Give us a holler when your trip is closer, and I'm sure we can find you some locals to take you out and show you some of our good sites.
