There are a few tried methods. It depends on how short your ramp is... if you roll your trailer wheels off the end of the ramp, it will not matter how long your trailer tongue extends. Your trailer will be a total pain to retrieve.
I would go determine the length that you can safely back the trailer down the ramp.
Do not get too worried about getting your rear wheels in the water if you have a vehicle capable of pulling your boat from the water. In the case of a 19' O'day, an SUV or minivan should be the minimum. You could back in til your tailpipe is just clear of the water.
So, if you still need an extension, google trailer extensions to see how people have built extension bars. If you only need a few feet, you can just get a few feet of 2" square steel and add a hitch and ball to the ends. More than a couple feet, and you are gonna need a wheel to support the ball end under the trailer hitch. People have also used extension straps to let the trailer down the ramp: block the trailer wheels just above the water. Attach a strap from the trailer to the vehicle. Unhitch the trailer. Pull the vehicle forward to tighten the strap. Pull the blocks from the trailer wheels. Slowly back the trailer into the water. Do not roll the trailer off the end of the ramp! Every ramp I have seen has at least a 1' drop off the concrete into mud. The force required to pull your trailer over the ledge will be tremendous.