If you discover that you are not compatible with your tablemates ask the maitre d' for another table. I've had to do that twice over the years (I did wait until the end of dinner). If by chance you see them, frankly it's none of their business but you can say that you didn't like the table location or as the other poster stated, lie and say you ran into friends and were able to be accomodated at their table.
When you check-in at the pier you are asked to produce a credit card. During the voyage you can go to the Purser's Desk and put cash on your account to handle your on board expenses so that nothing need be charged to your card. The charges on a credit card are not posted to the credit card company until the end of our voyage.
Sun Princess (2 times), Mississippi Queen, Dawn Princess, Coral Princess (2 times), Regal Princess, Grand Princess (2 times), Diamond Princess, Maasdam, QM2 (17 times)