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...