I used to feed Water rail with casters (pupae of maggots) purchased from fishing tackle shops by the pint, but always suspected they did not really like them that much. When conditions were cold and tough they would eat them, but with a lack of enthusiasm. These days I use the freeze dried mealworms sold by pet shops and garden centres for feeding birds. Water rails are far keener on these. I encouraged the bird to walk a small plank submerged under the water and built my props on top of it, placing the mealworms at the far end. They soon discover them and come to eat every 5 to 10 minutes posing well for the camera.
The site is very photogenic, but wet. To avoid lying down in the damp I used a right angled finder so I could sit on a stool and look down at the camera which was a ground level. It means being bent double in a very uncomfortable position when the bird arrives, but seemed better than getting soaked lying in the shallow water.
All taken with the Canon EOS 1dx and the 800mm f5.8 lens.