Its not that often you see seals that far up the bay... I windsurf a bit at williamstown beach and have only seen a seal once - it was a small one.

You do get seals hanging around on the bigger channel markers in the bay, usually they are at the bottom and western end of the bay- that seal could of been trying to get a feed on some penguins I doubt it will be hanging around otherwise they would make it a tourist attraction!

I think the chances of getting taken by a shark in the bay are pretty slim - different story if you were sailing in SA.

I think you have more chance getting injured by an outta control kite boarder in St Kilda. I have seen plenty of kiters colliding, tangled up, struck by falling kites and slammed into the bushes and the rock wall!