Who in the world decided their names? I remember which is which based on the sound. "Submersion" is "surjective", "immersion" is "injective". But really, the naming makes no sense. I can intuitively submerge a ring (a 1D object) into the ocean (a 3D object). so if anything, I'd expect submersions to be locally injective. You can only submerge $X$ into $Y$ if $Y$ is "larger" than $X$. The definition asks for the precise opposite!