There is a Flip Layout Effect.
You can set the Height of a Sprite to negative.
You can clone a Sprite in Classic.
The 'Picture Editor' for that cloned sprite has a Flip tool (icon in the top bar)
The erase tool can erase with soft edges (Hardness = zero), so you can draw out that coastline.
Most links to 3th party effects are dead, i suppose.
A subtle 'warp' could give just enough illusion of water.
Using the 'Image Manipulator'
Add 2 sprites, 1 'source', 1 'destination'.
Add the 'Image Manipulator' object to the layout (that is faceless)
Now, by instance ....
Under a 'Start of layout' ...
Add the action ImageManipulator > action > Copy image from sprite 'source'.
Under a 'every x ticks'
Add the actions
ImageManipulator > action > Apply any effect
ImageManipulator > Copy image TO Sprite 'destination'
That is the basic setup. But it is hard (for me it is) to get it really working, because the ImageManipulator takes 1 tick for its manipulations.
There is a Flip effect in the effects.
You can use any of previous mentioned methods on sprites added to an 'off-screen' Layout. Use Flip effect on that layout. And bring it onscreen with the Layout plugin.
Here is the manual for Classic:
Should bring you a lot more the right direction then i can provide to you.
Hope added enough 'presence' to my nose, after sticking it a bit 'thoughtless' in this topic.