A seaway bill is a receipt of goods issued by the ocean carrier to the customer (also called the consignor or shipper).
It is a contract by which the ocean carrier undertakes to transport the customer’s cargo in its vessel or vessels, from one point to the other.
It is a non-negotiable contract between the ocean carrier and the customer to deliver the goods booked by the customer to a specific consignee.
It is this non-negotiable nature of the bill that sets it apart from the regular bill of lading
https://www.marineinsight.com/maritime-law/what-is-seaway-bill-in-shipping/