I'm still impressed with how easy it is to get things done in the Godot engine code. What little official documentation there is on making editor gizmos is geared towards GDScript/GDNative, so I was pretty much on my own. However, doing a global search for "Gizmo" in the source code revealed that the CSG module had its own gizmo code, and provided an excellent reference for the steps to register an outside gizmo with the editor.
Drawing a gizmo is also easy, you just hand it a set of points representing the lines you want to draw, similar to using an ImmediateGeometry. There's more powerful things you can do as well, but I just wanted a way to visualize the position and orientation of the props and rudders.
I've also already got a way to simulate a singular directional current running through a WaterArea, so progress on my last Hydro features are coming along quite nicely. My flagship demo to showcase all the Hydro features together is really starting to take shape as well. It's nice to be able to bounce between coding and world-building tasks to make constant progress without getting burnt-out on either one.