So in the dictionary when you look up a word it will say 'see also' to tell you that the meaning of this word is the same as this other word so you can look that one up to get the description. See in this context is in imperative form and means look at. In this context marsh see also swamp means swamp = marsh. They are synonymous

I'm pretty sure they meant "sea" "see" is to look and notice something using our eyes The sea is the salty water that covers about three-quarters of the Earth's surface

so in the dictionary it will say 'see also' to tell you that the meaning of this word is the same as this other word so you can look that one up instead. See in this context means look at.