This is another one my father does if you have baitwells/livewells and the need to keep the water fresh and clean but don't want to use pumps.
Utilizing the existing fishbox drain tube, get a piece of flexible hose that will fit snugly inside it, add a piece of pipe with a cap and holes drilled (starting at the level you want to maintain the water height).
This works well with boxes that drain to the side or rear.
If draining to the rear, just add a 90 in order to reduce any engine exhaust problems.
For real troublesome bait like pogies or shrimp the addition of an aerator or bubbler greatly extents their survivability.