It is not compulsory so yes you can opt out of the captains formal dinner, most cruise ships have several other dining areas where you could eat that evening. If you are being put off by the possibility of being out of place by not having a tux or a dinner suit then simple wear any normal suit or smart attire. This is not always possbile on all cruise ships but you can ask.
There is no obligation to go to any of the formal nights on any cruise line I know of with the exception of Silversea and there you can dine in you cabin/suite if you want to avoid it and most ships have an alternative dining venue.....regards...Nei.
Thanks for answers mr mrs neil down and mr blyth from dundee.
You're very welcome Mr or Mrs McGuire and I apologise for the duplication which has happened here and on some others, I know I say a lot but ....enough said....yours.....Nei.