You need several different factors converging on a single coast for this to happen.
The first thing is that we need something that kills without discrimination and cannot be evolved away from. So most biological toxins are out. I propose undersea anoxic zones, or maybe volcanic activity.
Then we need something to push the sea life into it, since they won't tend to want to go where they'll die. We need some sort of undertow current that pushes them into it reliably (not an oceanographer, someone else may be able to provide the details for that).
After that, we need the sea life itself. Deep sea environments tend to be rather scarce on the ocean life... if you go to the bottom, you see fish and other life but rarely. So this won't be a deep part of the ocean, perhaps some giant reef system, and equatorial too. It will be within miles of the anoxic zone, but farther from this coast, so that sea life is pushed through to the coast.
Finally, the geology of it needs to cause extreme tides, so that this stuff is lifted onto the beach to some extreme degree.
With the right configuration the beach of death won't just have rotting carcasses strewn about, you might actually see some sort of geysers or bubblers out in the water itself.