I agree with Mike the sun does wonders for my "P"
but if he has topical meds with coal tar he should be careful in the sun.
I find the salt water actually helps with the scaling ...after a few days I just have red patches and no scales.
People are a lot more informed about "P" now than 30 yrs ago when I was DX.
I would just ignore people ...who cares what they think...if they ask or see them staring I just say "I have Psorasis" ...most know someone who has it and just move on!!
Tell him to enjoy the cruise and not to worry about what others think ...everyone has some problem