Joining a ship — "signing on" — is the moment your Seafarers' Employment Agreement (SEA) takes effect and you become subject to the Master's authority and to the rules of the flag state. It is also the last point at which you can confirm in writing that the contract, the wage, the ship, and the rotation are what you were promised. Take that opportunity seriously: once you sign on and the vessel sails, your leverage drops sharply.
MLC 2006 Title 1 (minimum requirements for seafarers to work on a ship), Regulation 2.1 (Seafarers' Employment Agreement), Regulation 1.4 (recruitment and placement), STCW Regulation I/14 (responsibilities of companies). The SEA must be signed by both parties, copied to the seafarer, and lodged with the flag state register.