Here are a few tips, hope they help:
- You should use LT to aim (that reduces the shaking and keeps your sight in the direction you want even if the boat is moving somewhere else)
- Use rapid fire with RT, you have unlimited ammo
- Don't try to aim perfectly. It's almost impossible to shoot every single enemy. It's more effective to move on quick, so you can get more kills in the next area.
- The red markers highlight enemy AREAS, there are mostly 2-4 enemies in one spot.
- A marker disappears if you kill all the enemies in the specific area. When you recognize that an arrow is gone, move on to the next one.
- If the boat reverses and the achievement didn't pop, restart your checkpoint.
Edit: If you're unhappy with my tips, feel free to comment or write me a message :)