Maybe they could grow that algae crap that can be used for food and all sorts of stuff.
As for defense, they'd almost have to incorporate carrier strategy...air power, missiles, torpedoes...surface and sub-surface screening vessels...could get really pricey...and even then not a guarantee for somebody who was really motivated to take it out just for spite.
Would not want to be that fat a target myself.
I would prefer a modest yacht in the 30-40 range, outfit/armed myself...something small and less detectable and less threatening-looking would be more my style.
Actually, my own sub would be the optimal SHTF sea-bourne vehicle!