It took about twenty minutes in the middle of the night to set up in the middle of a wintery pool, wellies full of water by the end of it. Five minutes later my flash, which I'd brought out solely to test, stopped working... If anyone knows about fixing Metz flashes, please message me! It still makes noises, and the button glows, it just doesn't flash. In this case "two out of three ain't bad" ceases to apply.
Anyway, having set up I figured I'd just trying gelling my flashlight/torch instead, and ended up with a shot I'm happy with and some neat lessons around lighting a scene.
Lighting is from a Lenser MT7 flashlight - first with a turquoise gel beneath the water (in a zip lock freezer bag), then with an amber gel aimed along the wall from the right, and vertically down on the scene from the top of the wall. I've included a flat white lit shot of the scene in the comments so you can see how it transformed things - without the lighting, this was pitch black bar a tiny sliver of moonlight on grey rock and brown water.
More to follow from the location in future. Hope every one is having an amazing Christmas/holiday run.