I took the tour in the summer of 2015 (and had a great time!) and was a little worried about seasickness, as I've had a couple of experiences over the years I brought dramamine as a precaution, but never needed it. As other posters here have said, the long rides (the ferry to and fro Aero--about 2 hours each way) and the overnight boat to Norway, are large and steady. The ferry to Aero is not unlike the Staten Island ferry and the boat to Norway is built like a cruiseship. In fact, with the overnight boat, I would agree that you really wouldn't know you were on a boat.
The cruise on the fjord is on a smaller boat, but it's still quite sturdy and I don't remember any rocking (and it was windy that day). I doubt any of these would be a problem. There are also two short ferry rides, one the first day in Stockholm en route to the Vasa museum and two and from the Bygdoy peninsula in Oslo. I don't think either of these rides is longer than ten minutes. If these two small ferries do cause some sort of problem, you can walk or taxi back from the Vasa museum (in fact, our guide recommended we do that for the experience). I'm not sure about Bygdoy, but worst case scenario, you would skip that outing. While that would be too bad, that's the day you're set loose with an Oslo pass, so you would still have plenty to do. With all the ferries, you have the option of staying outside the entire time (well, not so much with the overnight boat, but only because you'll want to sleep at some point!), and at least with me, fresh air helps a lot whenever I feel a little queasy.
Finally, you might contact the RS office and see if they could reach out to their Scandinavian tour guides. I can't imagine you are the first potential customer with this problem, and the local guides might have useful input.
I hope this is helpful and that you do decide to take the tour!