The one we are looking at is with P&O on The Ventura
I worked on the Ventura for 6 and a half months, so in regards to the formal wear they do have set formal night but you can glam it up as much or as little as you choose. Most guests will, so join them if you don't want to stand out. I always liked to see the odd rebel still in their shorts and flip flops amongst all the tux's!
The restaurants do have seating times for dinner, from memory 18.00 and 20.00. If you want to splash out a little I recommend the two posh restaurants, the white room and east. They are fantastic. There is also a buffet that is open for about 21 hours a day. Which is alright, but after eating there every day for more than half a year it got a bit samey!
Come and go as much as you like in the ports as has been said already, I never got to the Caribbean but the word was there wasn't a lot to do regarding excursions, more beaches and shopping. If you do a bit of research you can do the excursions yourself, but as said again, the ship won't wait if you are late. If you've not been to Madeira before, it's a great little island.
The other thing is, make the most of the entertainment! Get to the shows even if you think it won't be your thing, you'll be paying for it in the cost of the trip anyway!