Two divers raided shipwrecks off the Kent coast and failed to declare valuable items they removed from the wrecks, including bronze cannons.
David Knight and Edward Huzzey, both from Sandgate, Kent, admitted to 19 offences between them, the Maritime and Coastguard Agency (MCA) said.
The first known objects were removed in 2001.
The shipwrecks targeted included German submarines from the First World War and an unknown 200-year-old wreck carrying English East India Company cargo.
The MCA said the items included eight bronze cannons, three propellers from German submarines, lead and tin ingots, along with various other artefacts.
It is thought the combined value of the items is worth more than £250,000.
The MCA is aware from diary entries that Knight and Huzzey used explosives and sophisticated cutting equipment to free wreck material.
They will be sentenced on 2 July at West Hampshire Magistrates' Court in Southampton.
Alison Kentuck, the MCA's Receiver of Wreck, said: "Our message is clear: all wreck material found within or brought within UK territorial waters must be reported to the Receiver of Wreck. It is not a case of 'finders keepers'.
"By reporting wreck material you are giving the rightful owner the opportunity to have their property returned and you may be adding important information to the historic record.
"Legitimate finders are likely to be entitled to a salvage award, but those who don't declare items are breaking the law and could find themselves facing hefty fines."
Mark Harrison, English Heritage's national policing and crime adviser, said: "We recognise that the majority of divers enjoy the historic marine environment and comply with the laws and regulations relating to wrecks and salvage.
"This case sends out a clear message that the small criminal minority will be identified and brought to justice."
Mark Dunkley, English Heritage's maritime designation adviser, said: "The investigation has highlighted the need to tackle heritage crime, wherever it occurs, so that the remains of our past remain part of our future."
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