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The only problem that i can see is that on some very few ports of call you go ashore by tender this is a small boat that you get to by going down steps on the outside of the ship if you avoid these cruises you will be OK.
You shouldnt have any problems as most ships now have an integrated gangway where you cant see any empty spece at all. I am sure the staff will be too happy to help you board without aggravating your phobia, go for it! cruises are wonderful.