There is a set phrase, "from the mountains down to the sea" to mean "encompassing every part of the land". Would this be better? "People mountain people sea" is broken English.

人山人海？ I don't think this chengyu translates very well into English "word for word". Although you could say "a sea of people". A common English idiom of rough equivalence might be "to be packed like sardines". Eg. We were packed like sardines at 8:30 this morning on the subway.

Do you mean People from the mountain, People from the sea? as in describing where people are from? Not sure what you mean yet to give you a better response.