Strainer for drains that doesn't stick out.  Prevents debris and small objects from accidentally getting lost.  Protects drainage hoses from clogging.  Designed to be fine enough to prevent any but extremely small creatures from getting through.

The optional o-ring holds the screen in place, minimizing the chance of it coming out, as well as making upside-down installation possible (e.g. underneath a catamaran's bridgedeck).

Angled at 30º, the foil-shaped separators (potentially) trigger a vortex for faster water flow, while also (possibly) creating low pressure at the entrance, further increasing the performance.

Free, open-source, parametric.  Designed using ShapeScript.

Printing

Rigid UV-resistant filament would be the best choice.  Use 100% infill to ensure it doesn't get lifted out of the drain in case of an overflow.  0.2mm nozzle would work best, but MSLA is ideal for this design.  Minimal layer height.