A snake can't get through a solid buildup of sea water
calcium carbonates...you need ot dissolve it. Some people advocate removing the hoses and beating the bejabbers out of 'em on the dock
to break up and remove the buildup...if you're gonna go to all the trouble to remove 'em and put em back, replace 'em!
A 12% solution of muriatic acid should dissolve it...flush it down the toilet...go away for an hour, then try a bucket of water. It may take more than one application to dissolve it all.
Once you've opened up the line, now you want to prevent future buildup. That's easy: flush a cupful--two at most--of undiluted distilled white vinegar ALL the way through the system once a week. Wait an hour, then flush a quart of water through.
Also flush a quart of clean fresh water once a day to rinse out the hose.