On our 7 day Western cruise in January I decided to take a head count of the men in our dining room (second seating) on formal night. The tally was roughly:
suits or blazers with dress pants + shirt & tie - 75%
polo shirts or long sleeve shirts (with pants I'm glad to say) - 20%
tuxedos or dinner suits - 3 %
beach wear (cut-offs, slogan T-shirts, etc) - 2%.
DH lugged along both a suit and a blazer with dress pants, and three dress shirts and three ties. They were all worn (formal night, semi-formal night, Palo's) but he said if he were to do it again he would only take a navy blazer & grey dress pants and perhaps two dress shirts. As a matter of fact, we've just booked a cruise for next January (4 day Wonder, this time) and he insists that he's not even planning to take a blazer on this one. The jury is still out, but I may give in gracefully.
Conclusion: If you had really wanted to blend with the crowd on formal night, the business suit or blazer would have been the way to go on our cruise. The men wearing tuxedos all looked very nice, but it was by no means necessary to buy or rent a tux especially for the cruise.
Hope that helps, Uncle Al.