Is this the plugin?
You can submit an issue on the github tracker. However, the two issues there were posted almost two years ago, and never received a reply.
The author provides his email address in the README.md file.
Which version of Godot are you using?
I installed the plugin in a test project in Godot 3.5.3-rc1. I was able to fix that error by changing each "preload" to "load":
add_custom_type("dynamic water", "Node2D", load("res://addons/DYNAMIC_WATER/dynamic_water.gd"), load("res://addons/DYNAMIC_WATER/icon.png"))
Do you not get an error message telling you to do that?