Not just marine iguanas but all do this. It's a way to excrete salts from the body. It shoots out of their nose like a sneeze and they may do this several times a day. read more
In the case of iguanas and many other lizards, sneezing is how they rid their bodies of certain salts that are the normal by-product of their digestive processes. This does not mean that if you do not see your iguana sneezing you should be adding salt to his diet, nor does it mean that the foods you are feeding him are too high in salt. read more
Some marine iguanas are so big because of where they live. read more