One of the primary advantages of a well designed sea chest has to do with the potential ingestion of debris.
One principal reason debris is ingested by a typical through hull is the volume of water being sucked in by a very (relatively) small opening. There is a large vortex in the water causing surrounding debris to migrate toward and eventually into the through hull opening.
With a well designed sea chest, there is a very large (relatively) volume of water available for the through hulls inside the chest. The large opening at the outside of the hull has very little water flow caused by the individual through hull intakes inside. Therefore, there is little to no debris being drawn towards the intake(s).
In the photo below, you can see that our through hulls do not have any strainers on them. There is a only single "grate" type strainer on the outside of the hull over the entire sea chest opening. The top of our sea chest is about 10" above the waterline.
The boat has operated in this way (no individual strainers) through several owners, over 25,000 miles, and 4000 hours of engine time without any issues related to ingestion of debris through the sea chest.
JOOE,
Ray
(.....Just One Owner's Experience)