These diesel heater fuel tanks are non-compliant for marine boat use because they are not certificated for such, as you correctly supposed this is related to certificate for fire retardation. In all situations tanks of black/blue colour should be chosen rather than clear or opaque colour to minimise ultra-violet light degradation of the diesel fuel. Hope the above helps. Butler Technik have a heater repair and diagnostic service available should you choose if you wish to learn more regarding reason for early failure of your D4 Airtronic heater. Please email sales@butlertechnik.com should you require any further information on above
Thank you
David