It's a metaphor for feeling lost and not sure what to do
Something is lame
It's a big, significant change in something; a paradigm shift.
a sea wall is something that helps keep high tides and hurricanes from coming in
Landsickness
I means, "I hope it's something good."
Sea creatures (i think you mean animals) are found in all parts of the sea. However, the pacific ocean holds 1/3 of all ocean wildlife. well mostly the bottom in the deep sea
This dream suggests that you feel that you are very deep in some issue or troubling situation. Being beneath the sea could represent the idea of feeling very much "over your head."
The term maritime means that something is related to the sea or to maritime shipping. The word can also mean that something is likened to or resembles a mariner.
something that dwells in the sea
something that lives in the sea
Mean sea level (MSL) which is the average level of the sea. Determinig "sea level" is a very complex measurement. All other altitude measurements in aviation, in one way or another, are referenced from the mean sea level.
It's not an idiom. It means that something is underneath the surface of the ocean, underwater.