Yes it's hard to get good chairs, especially during days at sea. Don't plan on sleeping in on these days or you're going to have a lot of trouble finding a seat anywhere, and forget about getting multiple seats together. I swear some people must get up at 6am to go get seats by the pool. As VA_Cruiser said, a lot of people do save seats. I've even done it myself (I'm talking saved one or two for 10 min or so since I was just the first one to the deck) but people will save seats all day just because they got good ones and don't want to give them up. And I've also heard about alot of people losing towels, sunscreen, belongings when they leave them on a set and come back to find them gone, so be wary.
5/24/2008 - Carnival Inspiration
5/11/2007 - RC Sovereign of the Seas
5/14/2006 - Carnival Elation