About Giant Sea Bass from Milton Love:

"First, folks who have looked at gsb food habits have found lobster remains, so this was not a one-off event. Regarding how it gets the lobster into its stomach - it is likely that gsb have pretty substantial pharyngeal teeth that likely help it crush anything coming down that way before it gets into the stomach. Fishes that concentrate on hard shelled prey (not gsb) like sheephead have pharyngeal teeth that are like paving stones, for instance.

Regarding salt water ingestion - salt water fishes have a chronic problem with salt waters ingestion and with salt water bathing their gills. They have two mechanisms to emit the salt that enters their bodies. First, their kidneys are designed to retain water and force salt out. Second, on their gills are "chloride cells" that also emit salt into the water.

Lastly regarding digesting the shell material. I don't know about this - it is quite possible that, assuming the shells are crushed pretty thoroughly in the pharynx, that material does not have to be digested, but is rather passed into the intestine and out the anus. That is the case with sheephead, for instance.