OK, I want to create a romantic evening - everything is organized except for the perfect beverage! For various reasons, champagne is out, so is any type of wine or beer or cider. Thus the question, apart from all the above, what can I serve for my romantic evening?
The most romantic thing is to conform to the desires of your companion. It sounds like there are specific problems with sparkling and red wine. Personally as much as I like beer, I don't think it is "romantic" with the possible exception of a Belgium Tripel. These taste great, are surprisingly alcoholic and look neat served in the appropriate glass (a large wine glass may suffice). What I do think might be fun is a good French rose (a sparkling rose is really romantic, but out). Otherwise investigate cocktails. Unfortunately, I not much of a cocktail guy so I don't have any suggestions. Googling "romantic cocktails" provides a lot of hits.
It's unclear what the restrictions are so i have to guess.
So how about highballs/longdrinks? If you are easy on the spirits it should be possible to drink them as beverages.
What might be a nice companion to food (depends on the food and your taste) as it's not too sweet and not to much alcohol: Lillet (white vermouth) and Wild Berry (Schweppes).
Margaritas are also common during dinner, but that might be a problem as you're on a boat.
Coke & rum, vodka & orange-juice, gin & tonic might also work.