There are 2 different endings in Coast Guard. I recommend attempting to play the game through on your own first and see which ending you get. The majority of the achievements are unlocked through the main story and only a few are tied to a particular ending. Get both endings to 100% the game.

If you cannot figure out how to get the other ending, I have outlined the steps in this guide.

Note: This is not a walkthrough as there are plenty of videos out there. Here is a good one if you’re stuck and need to see how to do something. In this walkthrough he gets the “Larry’s Alive” ending:
https://www.youtube.com/playlist?list=PLczqgWktoK-4VY_hLqLJFeJsbnlXUUGaf

At the point in the story where the oil rig catches fire and Finn attempts to save Fatima, but she is captured on the Ghost Ship, there is a decision to make. Will Finn save his comrade Coleman or will he pursue the murder suspect, Sam?

No need to replay the entire game! Load up a saved game, log entry at the Oil Rig Blowout chapter, make your way through the Generator Room, and continue on until the cut-scene shows Fatima is captured. Now, choose which of the two endings you will get:

Radio Larry and tell him to save Coleman and then chase down Sam. This will open up additional chapters about his secret hideout and lead Finn down the path of uncovering the whole story. Continue through the story and many achievements will be unlocked, including:

Rescue Coleman and bring him to safety on the Defoe. This will prevent Finn from uncovering the whole story. Continue to play through the rest of the game. Achievements for this ending are:

True Coast Guard Expert
You've seen all the ending sequences!
This is rewarded upon unlocking both of the above endings.