Speaking from experience I can say it is not likely you will find a ship sailing from Rotterdam to Caracas unless it was part of the Shell fleet of tankers. You should look for a ship from Netherlands to US where the person would most likely have caught a plane to Caracas...maybe not in 1939 but certainly from late 1940s onward, but travel to Venezuela would likely have been from a US city. FamilySearch holds aircraft manifests for travel from Venezuela in this period.
