Well, this sounds like a great adventure you'll have.
Quite a bit has been said here and all very good info. Jack covered the rapids east of Campbell River and around the outside of the island as well as others, so I'll just touch on Seymour Narrows and a little more about Johnstone Strait.
Tackling Seymour Narrows from the south:
If you see a tug, watch him, he always knows the best currents and back-eddys. In any case, round Cape Mudge about an hour and a half before slack at Seymour Narrows and hug the east side to catch the strong back eddy that will push you up past the Native village on Quadra Island and past the Govt dock. Then pull back out into Discovery Passage and head for Race Point. If your staying in Campbell River or Quathiaski Cove, then leave an hour before slack but in any case, keep an eye on the wind over the current to find the rest of the back eddy's to help you along. The current wanders quite a bit when you get past Race Point but generally once you round Race Point leaving a wide birth, (you'll begin to see wirlpools here) head for the Maud Island light and enter the Narrows keeping to the east side. You will likely see lot's of tugs/tows, cruise ships, fishboats etc. which will be going faster than you so dual watch 16 and 71(traffic) if you can or at least stay on 16. It's not a big deal going through here but you should time it properly so you hit Seymour near a change to ebb to push you north. Having said that, sometimes there is relatively little traffic and I have gone through under full sail with no problem.
The weather can pipe up pretty good here and it can change instantly anywhere along the strait, so you may want to listen to W1 closely. The mountains along Johnstone Strait create a funnel effect and with this against the tide, particularly around Kelsey Bay it can be steep seas. If it's heavy westerlies and big seas giving you little headway and an uncomfortable ride, you can take Nodales Channel or go up through Blind Channel and zip zag up the back way and come out Sunderland Channel you'll likely save fuel and time by doing this. Robson Bight just before Port McNeil is a great place to see whales but you should see lots along the way anyway, so make sure you have your glasses out. You could grub up in Port McNeil because you can walk to the large grocery store, if you wait for Port Hardy, you'll need to take a cab.
Alert Bay is an excellent rustic stop with a real nice museum with a large collection of ancient native masks and such. Alert Bay has quite a history and was once a very large fishing community. You can anchor in the bay or use the Govt dock. Sointula is also a must see, you can anchor in Rough Bay with good holding but remember, it's called rough bay for a reason. Sointula has quite a history to it, very interesting and it has a museum that must be seen for sure. You can fuel up in Port Hardy and head for Bull Harbour to wait for the right time to cross Nahwitti Bar.
Here's a good read for crossing Nahwitti Bar.
I haven't gone around the outside but the others here seem to have that well covered.
Another good read - Around the Island
Around Vancouver Island: A story about circumnavigating Vancouver Island, British Columbia