The BEST way to travel from Trapani to the Egadi Islands (Marettimo, Levanzo and Favignana) is by Siremar car ferry, rather than by the faster, more frequent and more expensive hydrofoils (operated by both Siremar and competitor Ustica Lines).
Both the ferry and the hydrofoils depart from Trapani's dock, which is within walking distance from the Cathedral and historical part of town. There are also frequent long-distances buses from the Port to Palermo (Segesta Lines).
The crossing to Marettimo by ferry takes 2.5 hours, compared to one hour for the hydrofoil. Less time is needed to reach Favignana and Levanzo. The ferry runs daily year-round, with additional crossings during the summer. Sunday schedules are slightly different. During the peak of the summer season, Siremar adds supplementary crossings to Favignana with a smaller fast ferry (which affords no benefits apart from a shorter trip).
The one-way fare to Marettimo in law season by ferry is 10.60 Euros; by hydrofoil 15.50 Euros. Fares to the other islands are lower.
Travellers with limited time can take one of the numerous hydrofoils to and from the Islands.
Aside from the fact that the ferry has much greater stability and seaworthiness than the much smaller hydrofoils, the ferry has the additional benefit of affording passengers wonderful panoramas of the offshore islands from its open-air top decks, ideal for taking videos and photos. This is not the case with the hydrofoils. In case of bad weather, the ferry is also much less likely to be cancelled.
HINT: Take the morning ferry to Marettimo, and return to Trapani mid-afternoon with a hydrofoil. That gives about three hours to explore the island (since the ferry turns around quickly).
Here are two nice photos of the Siremar Lines car ferry which sails to all three islands from Trapani:
The large windows show the location of the main indoor salon, with nearly 400 vinyl, non-reclining seats. Behind this salon is an open deck area with hard plastic benches. There is another, smaller, open deck above the main indoor salon, with a short promenade deck. The bar is located in front of the main salon. The three large windows under the bridge is the officer's dining area. The ship (in service 1985) measures 233 feet in length, 46 feet in width, displaces 1494 tons, can transport 800 passengers, 70 autos, and travels at a speed of 16.5 knots.