The Greek verb ekpleo is a compound of ek (out/from) and pleo (to sail), meaning to sail away or set out by sea. It appears three times in Acts (15:39; 18:18; 20:6), all in the context of Paul's missionary sea voyages, marking his departures from various ports.
The three occurrences of ekpleo in Acts trace the advance of the gospel by sea. In Acts 15:39, Paul and Barnabas separate over John Mark — yet the result is two missionary teams, not one. In Acts 18:18, Paul sets sail for Syria after establishing the Corinthian church. In Acts 20:6, he sails from Philippi after the Passover season. The sea voyage motif in Acts is providential: the same seas that threatened Paul (2 Corinthians 11:25-26) served as God's missionary highway. The gospel traveled the shipping lanes of the Roman Empire — ekpleo by ekpleo.