For the coastal waters, I can imagine some scenarios where water would intrude into the main companionway in a capsize, wave breaking over the transom, or even complete rollover. Thus, the requirements for a secured hatch. I am also reminded, that a secured hatch keeps things from leaving the cabin.

For the SF bay, I can imagine a boat on its ear, chute in the water near a rounding, someone hits you and spins the cockpit into the chop or the collision helps roll the vessel past 90 degrees.

For SHTP, I also used a rubber/poly shock cord that allowed me to keep the hatch tensioned down while secured to the sliding hatch.

I prefer thick polycarbonate so I can also see what's going on while I am downstairs. For the ocean, I also take a second wooden hatch with me that doubles as a portable nav/chart table.

Some like a 2 piece hatch that comes up to the height of the seats. If you take a breaking wave on the transom, this will help reduce the amount water that gets flushed in the cabin. I will take some pics and post.

:: It was before my time, but I heard the Racoon Straights
:: sinking was due to water going in the open front
:: hatch...
::
:: Maybe someone is knows for sure can chime in...