Old as Dirt!
You may have a colony of barnacles/oysters/mussels growing in your hose. If you can get at the through-hull and temporarily block it from the outside (without freezing yourself to death), a dose of Barnacle Buster/Rydlym or a similar descaling agent for a hour or two may be all it takes to clear your line. BTDT. It worked for us.The scupper is flowing much better after using the shop vac. I didn't get anything big out of it, just some debris, so there may still be an object lodged in the hose but it is draining now.
Thank you for the idea!