An investigation is underway after the discovery of a man's body on a boat in Nelson.

Tasman Police say at this stage the death is being treated as unexplained after he was found on a vessel moored in marshland opposite a skate park at 10.45am today.

State Highway 6 was now closed between Trafalgar St to Atawhai Dr and diversions were in place as detectives tried to work out what led to the his death.

Police were appealing for anyone who may have seen any suspicious activity in the area overnight or this morning to contact them on 03 546 3895.

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People could also phone CrimeStoppers on 0800 555 111.