Here is one good reason to get a passport.
If there was an emergeny at sea, and you ended up being taken to one of several countries which require a Visa to enter, without a passport, you're in trouble.
If you arrive in a country which requires a visa without one, if you have your passport in hand they will genreally issue a visa on the spot and then you are free to leave. (Brasil is one exception, the visa must be issued in advance, although I'm sure they ahve provisions for emergencies)
If you have a US passport but it is not with you, the US consulate will assist in emergency procurement of a replacement passport, and you will then be issued a visa and can go about your business.
If you don't have a passport, you've got a big problem.
You see they can detain you until you get the passport and thus the visa. Getting a US passport when you don't have a valid one and aren't on US territory is very difficult. The consulate will try to assist, but it still takes up to two weeks. During this time you can't leave, because you aren't there legally to begin with.
There are quite a few Latin American countries which require a passport and in some cases also a visa, Belize, Colombia, El Salvador, Guatemala, are just some of the countries requiring a passport. The Dominican Republic doesn't require it, but strongly urges visitors to have one.
That's enough incentive for me to spend the money on a passport.
Considering that in the near future a valid passport will likely be required to return to the US, it's probably not a bad idea to just bite the bullet now and get them.
Anne