I saw that people were a little unclear where they could launch a boat in Venice and thought that I'd provide a few locations to help out.
The closest to Venice Beach by way of water would actually be the small boat ramp at north jetty. It's actually considered to be Nokomis, but if you launch there you just have to exit the pass and turn south to get to Venice. Be warned that the ramp is very small and not really paved so it's probably only good for smaller boats.
If you are on the island there is a boat ramp on the way to south jetty, it's at the yacht club and is open to the public. From there it is a fairly short trip around to the pass and out to the gulf.
If you are right off of the island there is a ramp and the linear park which is right off of the east bridge. Off of the the north bridge, there is a ramp beside Florida West scuba. Off of the south bridge there are a few ramps in south Venice, but I would recommend against them because you will have a long trip up the intercoastal.
That should help out a little for now, if you have any more questions about a place to launch in or near Venice let me know.