former 50 Years a Software Engineer now in a hammock
Answered Feb 17, 2020
, MN, is 2038 miles from the Atlantic Ocean. Another answer mentions
OK, but it is only 1162 miles. It might be a contender if you included the Gulf Intracostal Waterway, but I am not sure the Intracostal is deep enough for ocean going ships.
The obvious contender is the Amazon River which is navigable for large ships for at least 900 miles to Manaus and navigable to Iquitos , Peru, (2250 miles) for ships up to 18 feet draft weighing 3000 tons. I dont think that qualifies as a deep water port.
The Nile is blocked by the Aswan High Dam, so it doesnt make it. The Murray River is navigable for only about 1200 miles, way short.
The Volga River system is a interesting one to consider. The river plus some additional canals goes all the way from the Baltic Sea to the Caspian Sea , a distance of about 2300 miles, of via different canals to the Black Sea.
20Geo addition: This site might help you determine the correct answer