Diving near Portland is pretty non-existent. Most people do weekend trips up to the sound. The closest place to just get wet is Klineline Pond or Battleground Lake in Vancouver or Lake Merwin east of Woodland, WA. The closest salt water is in Garibaldi out on the coast about an hour and 45 minutes away. The diving there can be a bit intense with ripping current and rocky entries. You have to plan your dives around tides and weather but it can be rewarding. I've done some beautiful dives on the Tillamook Bay jetty and some rather dark, scary ones.
The open ocean offers some good diving, mainly in summer with places like Cape Lookout, Three Arch Rocks and Cape Falcon but a boat (live in most situations) is required and the ocean itself can be a bit tempermental. There are also Pinnacles and rock walls in the ocean down by Newport and Depoe Bay but I have never dove down there. And last but not least, there's Netarts Bay. Boring but not a bad way to spend a day.
There is one spot in the Columbia I've been thinking about for years out by Rooster Rock but have never done it. Maybe its time now?