If I fill an iron container with plastic foam and throw it into the sea, will it float? Suspended? Still heavy? thank you ..

Of course it will float. I learned it when I was in junior high school physics. The container is about 30m3 in size and weighs 2.2 tons. The density of plastic foam is 35 kg/m3. The weight of a full container is 1.0 15 tons, and the container weighs less than 3.2 tons, which is far lighter than the weight that it can discharge boiling water, so it will definitely float on the water.