That's the main reason. Also marine archeology is expensive. I once heard an archeologist saying that if the rests have passed centuries underwater, one more is less harmful than looters.
Underwater sites are particularly harder to protect from looters than above / underground sites. If the stakes are high enough, scuba diving is a reasonable option for the criminally minded.
It wasn’t long before Costa Concordia was looted for its treasures.
Passengers possessions - e.g. jewelry, watches. Technical equipment on the ship. Items from the on-ship shops. Interesting artefacts (ships bells are often a prized loot from wrecks).
It wasn’t long before Costa Concordia was looted for its treasures.