If this product doesn't exist, it should...
My idea is to have an inflatable type of pad that would stretch out into the water. It would have 2 tube style floats that increase in size as they start from the furthest end and narrow as they get to the attach point.
Different density/pressure air bladders would be used to asist the dog up the ramp until they get to a a "treaded" step surface that helps them make the final transition back to the platform.
Tethers would keep it pointing out the back so it stays accessible.
If that fails, and you just want to know if the dog went in, you can get one of those "turtles" they use for kids when they are around pools unsupervised. This unit sends a signal to a "base" unit when the turtle gets wet and activates a sensor in the turtle that the kid wears.