A contractor working for TS Rigging was converting a barge from a commercial vessel into a luxury houseboat when he fell from a ladder on 4 April 2017.
Chelmsford Magistrates' Court was told that the ladder provided unsafe access on to the boat and twisted as the worker descended.
He fell 6 m on to the dry dock and sustained a collapsed lung and head injuries as well as multiple fractures to his neck, back, ribs and hip.
The Health and Safety Executive (HSE) said a gangway in the boatyard would have provided safe access but TS Rigging had not installed it because it was under pressure to complete the project on time.
Its investigation found the company had not carried out a risk assessment and had failed to implement measures to prevent workers falling from height.
TS Rigging pleaded guilty to breaching reg 6(3) of the Work at Height Regulations. The company has been fined £34,000 and ordered to pay costs of £5,917.
HSE inspector Nikki Hughes said: "This incident could have been prevented if the company had implemented a safe system of work to prevent falls.