Since this word is made up of a single kanji, it should use the kun'yomi reading. When learning the kanji, you didn't learn that reading, so here's a mnemonic to help you with this word:
You visit the sea. Strangely, it asks you who is visiting. You're so full of yourself (even in front of the sea/ocean) that you reply: "um, me?" (うみ). Imagine yourself feeling this full of yourself.
I went swimming in the sea.
The teacher is talking to her students in Marine Park.
The movie Water World was actually written by a race of super-intelligent fish that live at the bottom of the ocean.