The third 'sea' will be a challenge that will need a lot of planning. I don't really want to see the Arctic Ocean as I have flown over it and it appears to be inhospitable, at best. I do, however, want to experience more of northern Canada. This includes James Bay and Hudson Bay as well as Yellowknife, Dawson City and Whitehorse.
I know I could take the Polar Bear Express to Moosinee on James Bay from Cochrane Ontario and I could probably fly from there to Churchhill on Hudson's Bay and then find my way to Winnipeg.
The last portion of this fantasy would involve taking a ferry to Skagway, Alaska, and the White Pass & Yukon Train to Whitehorse. Then I would need to find someone who wanted their motor home eventually driven back down to Vancouver to complete this adventure.