I think a turtle (aka seahood) is a must for green water, spray, and driving rain.
I think a traditional sliding hatch is close to impossible to make waterproof. If tested with a firehose, some water is sure to get inside. The easiest and least expensive way to attain watertightness is a elastomeric/rubber seal that is compressed. That means redesigning the hatch; there are ways of creating compression with a sliding hatch but not with a flat slide.
Question is whether watertightness is really needed.
Have you seen the capsize test done by the UK magazine Yachting Monthly?
The boat they used had a front hatch that was waterproof because the dogs forced the hatch down to compress a rubber seal. I think that boat had a single large dropboard and a sliding hatch at the companionway. That's kept closed for the test. And you don't see much water entering there - the water enters from vents. See for yourself at: [www.youtube.com
After watching that, you might conclude that latching the chart table/reefer top, and securing the stove, the table, and the cabin sole access plates etc have higher priority than waterproofing the hatch. Not to mention needing to replace the vents on the Dorade boxes with deck plates.
On Z, we put in a dropboard at each reef and end with the hatch closed at the third reef/trysail. Z does not have a means of locking the hatch from inside - we decided against that. I've never contemplated a means of locking the hatch partway: just tasting the idea now makes me think that it's an unneeded complication. If there were some emergency that needed immediate closure, having to unlatch it partway before closing doesn't feel right.
Tom Unruh & Jill White on Galatea had a way of locking the hatch closed from inside. I've forgotten the details.
Just for the fun of it, here're Tom & Jill on Galatea coasting South Africa in the Agulhas current back in 2008: [www.youtube.com