They agreed to make ships to go to Tarshish, and these were built in Ezion-geber.
We are now drawing towards the close of the history of Jehoshaphat's reign, for a further account of which those who lived when this book was published were referred to an authentic history of it, written by Jehu the prophet (Ch2 19:2), which was then extant, Ch2 20:34.
Commenting on 2 Chronicles 20:31-37
Then Eliezer the son of Dodavah of Mareshah,.... A city in the tribe of Judah, Jos 15:44. prophesied against Jehoshaphat, saying, because thou hast joined thyself with Ahaziah; an idolatrous prince, with whom he ought to have had no fellowship, even in civil things, it being both a countenancing him, and exposing himself and people to danger: the Lord hath broken thy works; the ships...
To go to Tarshish - "In the great sea." - Targum. By which expression they always meant the Mediterranean Sea.