Hiya Braveheart
Usually the reason for this is that the water where the AC is sitting has a water-class value of 1.
By default, water-class-1 underlies all CFS2 land. Many times, even in the stock install, some of this water-class-1 will extend out beyond the stock coastlines into what is supposed to be "real" water.
Except for its slight brighter blue hue, water-class-1 looks exactly like water except that it interacts with the environment exactly the same as dirt.
If you you shoot bullets into it, you get puffs of dirt- not splashes. If you start a float-plane from it, you get stuck in the "mud'.
Water-class-1 presents a slightly brighter blue appearance than normal water-class-7 value- the shallowest in-shore default water-class.
Many times when new GSL style scenery is installed, or large changes such as Rhumbas masks are installed, a lot of water-class-1 becomes exposed along the newly created coastlines.
Try slewing a short distance away from the shore and seeing if the problem sorts itself out.
If you view the area in top-down view you should be able to see where the water-class-1 leaves off and the water-class-7 begins.
Hope this helps,
MR