Lots of cruise lines are offering a choice we have just got back from a cruise on P&O oceana and a lot of people by choice ate all there meals in the Plaza which has a casual dress code some have a smart casual minimum dress code read the brochures or rinf and chat with a specialist cruise agent ,Best of luck.
You could have a look at NCL and Holland America gets a plug for more leisurely; relaxing type cruises now part of Carnival which could be another option. No doubt others will contribute further.
Not the Holland America we know although havn't cruised with them for over a year. We will be cruising with them in a couple of months.
NCL Freestyle - completely do your own thing and with big choice of restaurants (some have cover charge)- we liked it although we don't mind getting dressed up!
Most floating resorts give a choice of dining options if you really can't be bothered with the formal approach. American ships are particulalrly good at more 'casual' dining. Personally I think you are missing out on a lot of the joy of cruising, but each to his own. Enjoy which ever ship you decide on.