It's something you would only want to explore approaching the slack before low water to get free diving depths. Plankton blooms, heavy rains, or days ending with "y" may spoil visibility which can range from 2ft to 25ft with 10-ish being pretty typical.

Lots of Portland based folks five Hood Canal and there's some rocky headlands you can explore. There's a few waterfront rentals around Hoodsport plus Mike's Beach Resort that cater to divers.

I really like cold water diving in the PNW but it's definitely an acquired taste that requires a really good understanding of local conditions to mitigate risk. The really cool stuff is in British Columbia.