I know a shutdown will have an impact on air travel. Initially only a small number of flights may be cancelled but if it goes on long enough the more and more flights will be affected and perhaps some airports will temporarily close due to the lack of air traffic controllers deciding not to show up without being paid.
Also, TSA agents may not show up so security lines could get significantly longer.
However, what is the impact on cruising? Other than customs for returning ships what would get impacted?