Yes I think 8 in a row would be excessive. As one who used to "love" sea days I had a personal reckoning on our recent Panama Canal cruise. The first 4 days out of Seattle were fine as everyone was fresh and we were still excited. The 3 sea days after Cabo were a little more tiresome (tired of seeing the same people ALL the time), but still not bad. Going through the Canal was of course the highlight of the trip and reason for going, so after that the trip was pretty much over for me. The port day in Cartagena was interesting for me only because we were docked at a container terminal. But then those last 3 sea days crossing the Caribbean and Gulf to get back home were a total DRAG. I just wanted to get home. And a phenomenon that I hadn't considered before; on a 16 day cruise you realize the ship's musicians have a finite playlist and you get tired of hearing the same songs over and over again. So don't get me wrong, I loved the trip, but I realized there is a limit to my enjoyment of sea days. I'm sure we'll never do that long of a cruise again.
Andy