Port Phillip Bay is the bay that Melbourne is built on the very northern tip of. The eastern side is mostly sandy beaches with Melbourne's southern and eastern suburbs built up against it, before turning into the semi-rural Mornington Peninsula. The western side is less built-up until you get to Geelong, Victoria's second city, and the Bellarine Peninsula at the far southern end. The two peninsulae almost meet (there's a gap of a few km, through which all of Port Melbourne's shipping passes) and a ferry operates across between Sorrento and Queenscliff. The beaches are somewhat idylic; the tidal range is rather small and the waves gentle, and the sand a golden brown colour, with relatively little marine litter or seaweed. The majority users seem to be families with young children, 'surf rescue' clubs, whose members appear to outnumber surfers by a considerable margin, and kitesurfers.
I seem to have started a project to walk round Port Phillip Bay, mostly because it gets me out of the house at the weekends, but also because it would seem to be a shame to waste a once-in-a-lifetime experience to see lots of coastline that nice. In time-honoured tradition I've started with the easy bits, doing Port Melbourne to Parkdale (but not in that order, and in three separate chunks) so far. There are photos at
http://jcsu.jesus.cam.ac.uk/~rpc25/phish/index.php?dir=2008/Bayside%20Walk and Wikipedia will tell you
more about the bay.