This is likely being done not for deterrence reasons, but as leverage during any future negotiations:

North Korean leader Kim Jong-un said his country was “equipping the Navy with nuclear weapons,” state media reported Wednesday, as he also unveiled plans to build 10,000-ton warships.
The remarks came at the commissioning of the Choe Hyon — one of two 5,000-ton class warships the nuclear-armed state launched last year — in the port city of Nampo on Tuesday, the official Korean Central News Agency reported.
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