Willy, as the locals call it, has an impressive history: the town was founded in 1835 before Melbourne itself and served as the city's main harbour until the turn of the century. Today, it's an upmarket residential suburb with a swankly nautical seafront strip, flanked on one side by a yacht harbour and the other by seafood restaurants. While small, it's quite scenic and on sunny summer days tourists flock in droves.
Williamstown, Williamstown Beach and North Williamstown stations are in Zone 1. Metro services from Flinders St take 30 minutes and run every 30 min or so. Direct services only operate at peak hours; at other times, take a Werribee line train to Newport and transfer to the waiting shuttle. The town centre is a 10-min stroll from Williamstown station.
Williamstown Ferries  and Melbourne River Cruises  run ferries along the Yarra River from Melbourne's Southbank (piers near Langham Hotel) to Williamstown from 3-10 times a day depending on the season. The trip takes 50-60 min and costs $12/22 one-way/return, and as virtually the entire trip is along the river, it's very smooth sailing.
Williamstown Ferries also has irregular services, mostly on summer weekends, to St Kilda, just half an hour away.
Williamstown is easily covered on foot.