You need some still or slow-moving water that's deep enough for them to dive into, with tree or bush cover nearby, and ideally some perches over the water like sticks or reeds sticking up.

Duck ponds, lakes, canals, and the bits where rivers get slow and deep are great.

You're more likely to see them on partly sunny days, specifically when the clouds part and the sun comes out, as that's when the fish will swim up to catch bugs on the surface.