They should pay the government of Nauru to rent their sovereignty. They can keep the "island" above sea level by building a mooring tower and tie seasteads to it. Their territory can be the 200 mile economic zone of Nauru.

Also, there's lots of healthy stuff that can be kept in dry form. (Fruits, Legumes, and grains.) They will also have the ability to keep things frozen. Chest freezers can be highly efficient. Especially if they are not opened very often.