Probably you were reading depth in fathoms (i.e. 6 feet). It is plenty deep everywhere, provided you don't stray too far out of the main channel. There is shoal to the left and right of the marked channel, but you kind of have to go sightseeing to run into it.
There is one thing that gets a lot of people. Look on the chart. You'll see the green buoy that you don't want to stray too far north of. This is pretty clear. However, immediately across from it, there is another red marker which many people mistake for the red buoy a ways to the ESE. You want to stay between the red and green buoys, as they mark the channel. Sometimes people get confused thinking the pylon with a bright red marker is the red buoy they need to stay north of. The red buoy is comparatively hard to spot, so people frequently go north/west of the pylon, but south/east of the red buoy. These people get stuck in eel grass or worse. I don't know if you get the Northern Light (newspaper), but there was a picture on the front page of a sailboat that made that mistake several weeks ago.
I know about this because I too made the mistake. Coming from the south, the red buoy seems too far west, so having spotted the pylon, one tends to not look all the way out to the west where the buoy is. Fortunately, I just bogged down in grass and easily freed the keel, but the lesson was learned.
Anyway, from your direction it is no big deal unless you decide to swing around to the south and come up from Birch Point. Then, just keep an eye out for that. In front of Semiahmoo Resort, the shoal runs farther out than you would think.
You should have SW winds, as these are prevailing this time of year. So, it should be a direct shot. And, of course, you can see your destination the whole time. It's a little tricky to spot the exact entrance into Drayton Harbor, but just shoot for where you think it is in the distance, and as you get close, stay between the buoys. By the time you get to the buoys, you'll see the entrance. If you're nervous about it, try to plan your entrance near slack tide because it can get pretty brisk through there.
Listen for the sound of a ship's horn as you enter the harbor. The Plover Ferry, as you probably know, runs foot traffic between Blaine and the spit every 30 minutes or so. They sound their horn when entering/exiting the breakwater. The captain is very friendly and won't run into you or anything, but that's what that sound usually means.
One more thing, if you arrive during decent winds (say 10 kts or so), consider going to Blaine instead. It is MUCH better protected, and you'lll have an easier time of it if winds are high. Their guest dock is much nicer too, with PLENTY of room to maneuver.
To get there, just turn left into the gap in the breakwater (between the two concrete walls). Proceed along the breakwater past the commercial vessels and you'll see some conspicuous shoal markers to your right along the breakwater. Immediately after these markers, turn left toward the flag and main office. It is really wide here. Guest and visitor moorage is all along the left. Just tie up anywhere along here and go check in at the main office.
There's a good chance I'll be there as well - over on the A dock - almost at the end closest to the main office. I'm out cruising right now, but I've got to get back to do a few maintenance tasks before sailing back down to Seattle in a few weeks.
Best of luck with the trek! Have fun.
