I have sailed the Horizon's sister ship Zenith many, many, times and Horizon twice long ago. We are saling her again on the repo next month and we are on the Atlantic deck. This deck is by far the most convenient deck to be on for getting to places. Through the years we have found that the stairs on both Zenith and Horizon really tire you out, I believe it is the spacing or something. My wife makes me use stairs all the time and I have no problem on any other ships. She is a health and exercise type person and even she says thoses stairs tire her out. Elevators are slow, so we found that the Atlantic Deck is a perfect location for heading up to pool or eat.
I get seasick or motion sick easy yet have cruised since 1984 getting ill only twice in over 45 cruises and that was in extreme weather conditions. It really would not have mattered where on ship we were. The other times I find generally ships are smooth sailing, go around storms anyway ( depending where sailing to ) and that starting out taking BONINE ( located next to dramamine at local pharmacys ) works well for me. Usually I don't even bother taking it after 2nd day.
Have a great cruise.
George in NY