Probably means never heard of a doctor having a seal? I've never heard of it either. If you really got nothing about it from the cruise line - which hopefully did not then charge you for the excursion? - definitely let them know this is going on. Both so they can notify people properly, and maybe so they can have someone who actually knows something about this work with their vendor to do something about it, because it seems bogus. (Or find a different vendor, though I don't know how many options there are in Aruba that could handle cruise traffic.) I agree posting on TripAdvisor is a good idea, and maybe a cruise site would be good - places people likely to run into this situation might read.
Unfortunately there seem to be lots of instances of dive shops/employees anywhere emphatically stating things as fact which are simply not true, for all sorts of reasons.
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If you poke around on the Red Sail site there is this -
http://www.redsailaruba.com/pdf/AR_Medical_Statement.pdf
Which does mention something about a seal, but it could have been updated since April?. Maybe it's an Aruba thing, but they have to know a lot of their customers are from the US.