Yeah, the problem is that the turrent behaviour by itself is made for games with a top-down view and doesn't take things like gravity into account, while you're using a side-on view and projectiles that fall - so your enemies are trying to shoot at the most direct path to the player, but the projectiles are falling straight into the ocean
It's totally doable though - you'll just need to figure out the formula relating distance to angle, then:
Every tick > set [enemy turret angle] to [whatever angle scores a hit at that distance].
In the attached .capx, I've created a text object that displays the distance between the player and the enemy ship, and the angle of the player's turret, as well as disabling the enemy movement. If you play around with it for a bit and make some notes you should be able to get a distance/angle formula. (You might want to switch things around so that you're controlling the enemy ship's turrets and the text displays the angle of the enemy ship's turrets, since that's what you're looking for - sorry I did this in a hurry on my lunch break, there's some assembly required )
Hope this helps!