I have several hundred dives in the San Juans. I believe the best for newcomers is at the county park on the west side of San Juan Island (Smallpox Cove). You can enter by the boat ramp and swim around either point at the opening of the bay. You can also scramble down the bank on the north edge of the park and explore those rocky out croppings. I occasionally swim out to Low Island just off the park and dive that. This is a place where you need to watch the currents carefully. With a rocky substrate you will see all of the attached invertebrates, feather stars, abalone, fish, seals, and orca. The whales are more easily seen from the surface; finding a submerged orca is a challenge. The winter offers better visibility and slower tidal currents during the day, but the weather is more of a problem. There are a couple of guidebooks describing dive sites in the San Juans.