Hi Brent. The groove for an "O" ring is quite a bit wider than the
"O" ring itself, leaving a space inside the groove for water to get
into. The "taper" in the groove is on the inside face and it's only a tiny
bit, under 5 degrees out of plumb. The bottom corners of the groove are
supposed to be rounded slightly. The Parker "O" ring "design book"
explains the function and has charts for sizes and many different types of
applications depending on what the "O" ring is trying to accomplish.
There's quite a few different types of groove shapes with some designs
based on a rotating shaft, sliding shaft, and a bunch of other
applications. The common function of all of them is to apply pressure
against the rubber and force it into a TINY crack.
The "O" ring gets pushed up against the inside groove wall as
the water pressure is applied. The "O" ring doesn't really deform into a
slightly rectangular shape until the pressure builds up enough to squash
the hatch down.
The groove needs to be wide enough to accommodate the "squared" "O"
ring and still leave enough space for the water to get in there so the
water is applying pressure to the "O" ring, forcing it up against the
crack between the two metal surfaces. It's a real simple design and can
withstand a LOT of pressure.
Your rendering shows the taper on the outside groove wall and the
rubber "O" ring is filling the groove. That won't seal because there's no
where for the water to get in there so it's actually acting like a flat
gasket.
The water would slip under the squared "O" ring because the pressure
is being applied to the gap and not "behind" the "O" ring.
Frank D.