Based on the wording no ship can dock in the US or sail in US waters until July 18, or the no sail order is rescinded, whichever comes first (and assuming the no sail order is not extended). It doesn't prevent a cruise ship from visiting other countries that have not locked down their ports.
So cruise lines could, in theory, be up and running in the summer, and maybe even departing out of the US. I don't think it's likely, but the possibility is there.