Hi Geeber,
I live in central Florida also. The allamanda that I am familiar with have either a yellow or pinkish flower. They flower freely. However, they are vigorous plants which will start leaning when the branches are full of flowers. Mine do well, because they can lean on my fence. Mine are about 10 feet from water, so I can't answer your question about water.
I keep mine about 4 feet tall, at that point, they start drooping from the weight of the flowers. However, the branches are about 6 feet in length. If you don't mind them leaning on your hedge, they would work fine. You also have to be careful when trimming them - the sap can irritate your skin. I have very sensitive skin, so it bothers me a bit. However, the plants are quite spectacular. If you haven't tried it, the Florida gardening forum on gardenweb has lots of helpful people.