Don't they have the right to substitute boats for any reason? We saw at least three different ship styles in use, in the early season. Of course the best views have a scramble for the chairs. On our ship, there were certainly more non-open-air seats (covered or indoors on the lower deck) than there were with direct, open views. But we got what we came for, especially because we wanted food and beer service. We didn't trample anyone to get there.
We did a day trip while still in a hotel. You don't mention if you're encumbered with luggage while doing a Rick Steves busy-every-minute schedule. That will hinder reaching the best seats.
I'm suggesting that you can't get this degree of precision in your planning. Luckily, you're going after the peak August vacation season. But that also implies smaller ships to save provider costs.