I have psoriasis and there are times it flares up fairly bad. I have it on the hands, elbows, knees, legs, and feet and some other areas I won't mention.
I don't know how severe your husbands psoriasis is but I know it can be a problem both physically and emotionally. You will find that people on cruise ships are extremely accepting once they know about you. They are there to have fun.
There are a couple of things to do:
1. The sun helps (in moderation) if his medication doesn't increase the chance of burning. I always find that my psoriasis becomes better when I cruise. It can be a mixture of the sun and just being more relaxed.
2. Keep moisturized and make sure he uses his medication, if he does use medication. It sure is easy to forget to put on topical medication when you are on vacation. If he takes oral medication, makes sure he also takes that. Also moisturize after going in the pool and ocean. Chlorine and saltwater raise heck with psoriasis. Chlorine is worse.
3. If anyone asks, just tell them that you have psoriasis. Most people understand and the ones that don't........... to heck with them. I think you will find very few of these people.
You can always do some cover ups if it is more severe. Don't let it ruin his vacation. If he gets through daily life with it a cruise is nothing!!