A drawback to copper tubing is that it can work-harden and crack from vibration. Recommendations are for seamless, annealed tubing with permanent flared connectors, and secure mounting. Old-time motorcycles used to use a loop in the line from the gas tank to allow some flexibility, but most people use a section of suitable rubber hose these days. The chilling water won’t be a fire hazard like a broken gas line, but if it has a through-hull fitting below the waterline, you could get a sinking feeling. A seacock on all through hull fittings is standard equipment.
If you come up with a compact, low-cost cooling system for the occupants of a boat, please feel free to share the design.