Hi, I am a US citizen and work in the US. My fiancé is a non resident/non US citizen that works on a cruise ship that come in and out of the US but he does not reside here full time. He also has never paid US taxes as he technically doesn't live anywhere however we do own a house in the US that we of course pay property taxes on. He is on the ship 8 to 9 months a year. He is concerned that after we get married he or I will have to pay US taxes on his salary. The country he is a citezen of does not require him to pay taxes there as he is only there 1 or 2 times a year at most. Is he right to be concerned about this? Thank you so much.

Could you provide more information?
Is the owner of the cruise ship American? Under what flag does the ship operate?
Where does your fiancé live when he is not on the ship?
Does he have a US visa?

The flagging of the ship is irrelevant. If he is on a work visa from his native country, he owes NOTHING in taxes as long as he not met the Substantial Presence Test (SPT) AND as long as the income is NOT from a U.S. source.

If the income is from a U.S. source, then he owes taxes on the money earned while INSIDE the United States irrespective of the SPT.

The SPT is calculated over THREE years. In the first year, he owes no tax as long as he did NOT exceed 183 days inside the U.S. AND the money earned is NOT from a U.S. source.

In the second year, you calculate the number of days to see if you met the SPT. If not, you then add one-third of the days from the FIRST year and see if you have met the SPT. If the combination of the two years does NOT exceed 183 days AND the money is NOT from a U.S. source, you owe NO U.S. taxes.

In the third year, you calculate the number of days to see if you met the SPT. If not, you then add one-third of the days from the SECOND year and one-sixth of the days from the FIRST year to see if you have met the SPT. If the combination of the three years does NOT exceed 183 days AND the money is NOT from a U.S. source, you owe NO U.S. taxes.

If, during the first, second or third year you DO exceed the SPT, in THAT year you would file a dual-status return and pay taxes ONLY on income earned while located WITHIN the United States. However, now you have established residency in the U.S. so for every year thereafter, you are considered to be a resident of the United States. As a resident of the U.S. ALL world-wide income is now subject to U.S. income taxes, regardless of where you earned it.