I wopuld not recommend one in the swimming pool but did you mean in the dining room on formal nights ????!!!! but .
If you don't have a dinner suit then any smart dark suit will do, if your suit is not dark then the decisiion is made for you, but just for the formal nights max likely to be 3.
My husband wears a suit on formal evenings and shirt tie and long trousres on all othr evenings he has never felt out of place no need for diner suits.
The official policy is two formal evenings per week. On some longer cruises it varies but there are always some formal evenings. If you really want to avoid dressing up you can always dine in the Secret Garden cafe on these occasions.