Public transport schedules will update prior to your visit.
Castellammare del Golfo is served by Russo bus service between Palermo and San Vito Lo Capo, AST bus service to Trapani, Tarantola bus service to Segesta, and Trenitalia service, but the train station is inconveniently located outside town requiring a bus to the station.
Although there's transport from Castellammare del Golfo to Trapani, the public transport to Marsala would connect via Trapani, and the schedules aren't the most convenient to San Vito Lo Capo.
Public transport ends fairly early in the day, so you would really appreciate the flexibility of having the car.
Park at Palermo's airport when visiting the city.
The Prestia e Comandè service between Palermo and Palermo's airport runs later than other public transport from Palermo to any surrounding town. You could then enjoy a very long day in Palermo, including dinner, which is generally considered late in Sicily by tourist’s standards, just beginning around 20.00. Then take the bus back to the airport, collect your car, and drive onward back to Castellammare del Golfo.
Vagabonda