I have been on a lot of cruises during Halloween week - and for the most part they do NOT have costume parties. However, you never know.
There will always be SOME people dressing up - and there will be a party, but most people at the party will not be dressed up - just so you know.
The exceptions are the singles cruises where they have coordinated parties - but you guys are not single. so.
Halloween is now the favorite holiday for singles cruises (and gay cruises).