It's very fact specific. Shore-based workers that are assigned skiff duty on Monday but are flagmen on the road or laying asphalt the rest of the week are likely not going to be Jones Act seamen.
Their attorneys will argue that they are so they can sue their employer rather than receive comp benefits but just being on a vessel does not necessarily make you a Jones Act seaman. Need to be more or less permanently assigned to a vessel (or fleet of vessels), regularly exposed to "perils of the sea", etc.