The shelf is not to be cantilevered off a vertical surface but fitted between bulkheads.
I am in a marine environment so avoid materials like MDF and mild steel. regardless of coating in a situation where it will wear. So standard household stuff is out.
My first trawl through the internet for stainless steel heavy duty drawer slides came up with this LINK
With a 352 lb capacity they should survive even a rough interisland passage with the dreaded Caribbean 2 step running. These are short steep waves and everything bangs about when you come off a wave.
I could fit them reversed so that when the lathe is pulled forward for use the slides are in the closed position and pushed back would be open.
But I like simple solutions and might start with stainless steel feet running over Formica.. I could always add the slides if needs be.
I will of course have some kind of positive lock for use when sailing.