Complex sentence: The current boat, named the "John Craig" after the site's original owner, dates to 1996; the vessel it replaced sustained heavy damage first when it sank under the weight of a heavy snowfall and then as a result of salvage efforts.
Split into simpler sentences: The current boat, named the "John Craig" after the site's original owner, dates to 1996; the vessel it replaced sustained heavy damage. First it sank under the weight of a heavy snowfall and was damaged further in salvage efforts.