The IMDG Code is the sea equivalent of ADR: it sets how dangerous goods are classified, packed, marked, stowed and segregated on board. It is published by the IMO and revised on a two-year cycle, in step with the UN recommendations.
For the forwarder, the practical difference from road lies in segregation between classes inside the container and the ship, and in the dangerous goods declaration the line demands before accepting a booking. A correct ADR shipment is not automatically a correct IMDG one.