Duncan McMillan climbed on the boat, moored in the harbour at St Andrews, and cut the ropes that were securing it to the shore on February 9.
He then ran off as the creel fishing boat, used to fish for lobster, crab and prawn, drifted off into the North Sea waters off the Fife coast.
Its owner was summoned the following morning after another fisherman spotted the boat "firmly lodged" under a footbridge near the St Andrews shore. The boat had suffered damage to its hull, keel and wheelhouse.
Fiscal depute Joanne Smith told Cupar Sheriff Court: "In the following days it seems the accused eventually made contact with the owner and admitted carrying out the act and offered to pay for the damage in full.
McMillan, 41, of St Andrews, pled guilty on summary complaint to a charge of vandalism.
Sheriff Charles Macnair QC deferred sentence on McMillan until next month.