If people start to play cards, knit, whatever, I can see a staff member coming up and in a very polite but firm tone saying "I'm sorry folks but we ask that passengers leave the food tables once they're finished eating." And then just stand there.
Pretty OT here but I've wondered why people are so 'hot to trot' to get on board. I know there are different kinds of ports but I generally seek out some restaurant whose menu sounds good and have lunch there before boarding, You'll have tons of the same food - albeit good - for the next 7/14/forever days.