I bought a few matching tie and vest sets on
Amazon for about $20 each and wear one of those with my black suit on formal night. I usually bring all 4 so I can coordinate with my wife. It's a much cheaper alternative than renting a tux.
We actually dress up for dinner every night on our cruises which is why my wife brought 7 pairs of shoes and 4 handbags on our last cruise which was only 4 nights!
I don't have the guts to ask her how many she brings on our 7 night cruises. But I will say, she always looks fantastic