Yes it does happen but more of a curiosity or accidental encounter than an attack per say as far as getting knocked over. They do investigate kayaks often. Spilling one is purely accidental.
here are a few examples of encounters on U Tube
http://www.youtube.com/watch?v=gv7xim_k1q8
a good one:
http://www.youtube.com/watch?v=oKn0A1A7 ... re=related
http://www.youtube.com/watch?v=WixhGEvJKaY
It happens more often than one imagines. They are big and very strong and one coming up under you can easily throw you from your kayak. They do not show aggression but childish curiosity. For all they know the hull of the kayak is another manatee and a play friend. As you can see in the videos they push each other around in a playful push and shove.
A family friend was in the ocean off Ft. Lauderdale at low tide thinking her husband had just swam between her legs so she sat on him only to find it was a manatee. She walked on water back to shore.
I encountered a group swimming along the shore off commercial pier and one swam towards me and as I floated on the surface he went under me and surfaced brushing my back. Water was murky I only saw a huge submerged cigar shape shadow moving toward me so needless to say I chit my chorts. Talk about stomach in throat. I realized it was a manatee after he surfaced along side me venting his snout. 3 others surfaced just out a ways from me. I was in 4 feet of water.
So in answer to your query DieselYak .. yes it happens maybe not too often but it does happen and those who doubt it just don't know! Your location on the Intracoastal along John Lloyd park is a frequented place for manatees especially during winter months. They love the warm waters of the cooling canal for the power plant.