My son is allergic to milk and for many things for him and out of respect
for him, we simply do without.   So, we don't buy any more sour cream, and
we make fajitas and tacos without cheese or sour cream and use veggies or
just plain meat instead.  We don't buy cream cheese or any cheese of any
kind.  We make cheeseless pizzas and they happen to still be quite tasty.
We do still buy yoghurt for my husband and daughter (they need those
bacteria!) but my poor son does without soy yoghurt due to price and lack
availability.

Coconut milk, if you get one that has more coconut than milk, will thicken
to a nice sour cream consistency in the fridge when poured from the can into
a container that's easier to use.  I have used it that way as a whipped
cream substitute on brownies or cake; I suspect that if you season it a
little on the savoury side prior to chilling it, it might make a reasonable
sustitute (but I have never tried that...).

After living over seven years like this, we got used to it.  I no longer
miss cheese like I used to.  I also suspect that it's healthier (less fat!)
and easier on the wallet.

But with the soy allergy possibility, I understand your frustration.

Do call the cruise line before you leave and then contact the kitchen after
you board.  You cannot be too careful in this regard.  You could also ask if
you could bring along some of your own food.