In the San Juans, currents control everything. We generally try to start a dive at or just before slack current, but it isn't always really slack. Some of the best diving we've done is some variation on one team diving while the other team drifts along and follows their bubbles in the Zodiac. Or sometimes even in the big boat. Teams switch about the time the current changes, and we all drift back the other way.
Two times I remember when things went wrong-ish.
1. Trying to drift dive Deception Pass. This is a spectacular but advanced wall dive that is only possible at slack current. Well, slack didn't really happen that day, so it was a pretty rapid drift dive. Except I got caught in a sharp downdraft. Like somebody pushed the "down" button on an express elevator. Finning and inflating wasn't stopping the descent. I was just about to start trying to jam my dive knife into cracks in the wall when it spit me out. Then I was worried about getting in an updraft. Oh, and my second stage iced up and started free flowing... OK, only dive Deception Pass on a real slack slack.
2. Group of four ended a drift dive near the end of an island. As planned, the current started washing the party back toward the boat. Except I was just far enough around the point (really only a few yards) that a different current started washing me toward... Canada. By the time we mutually realized that I wasn't with the group any more, my head couldn't be seen in the surface chop. This was a bit before inflatable sausages came along. Boy, did I think of this day the first time I saw one of those. It was a lot of work to swim to the back side of the island and hike back across to where the boat could see me. They were looking in completely the wrong direction.