A defence system for a marine asset such as a ship or an oil rig. The system comprises unmanned defence units, each unit comprising at least one weapon and being deployed with respect to the marine asset to create a defence perimeter at least partially around the marine asset. The defence units may be submersible and carry lethal or non-lethal weapons. The units may be self-propelled and may be capable of positioning themselves with respect to the marine asset and maintaining a desired position with respect to mobile marine assets.
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