I guess I must be taking the right cruises because the open seating area is rarely filled at breakfast - at least for early birds. As a single traveller, I relish the opportunity to share a meal with a stranger or two. What better way to start a conversation? I often share a seat just for the sake of company, even when there are empty tables around.
Lunch is a little harder. Singles sometimes have to wander to poolside tables just to find an open seat. As far as reading at the table goes, I was taught it was bad manners to do that and put down the paper when someone joins me. If I'm the latecomer, I ignore it. But if it was the reader who joined me, I'll excuse myself soon as I'm finished.
The one thing I find rude, however, is someone sitting alone at a table for six without a tray who informs me that the other seats are 'reserved'. The idea of saving space for others who are still going through the serving line seems pretty inconsiderate. It's like the kid with the empty shopping cart in the grocery checkout who is joined by his mom with a cart full of merchandise. To me, that's akin to 'line cutting' and a major faux pas.