less confusion 1) A drytop will keep water off your torso and out of your boat if mated with a good sprayskirt. As long as you roll, it’s great, but by itself it proides no immersion protection if you swim.
2) You don’t need drypants. They can be nice for protecting you from the water sloshing around inside your boat, but a drytop/drypant combo will usually start to leak at the waist if you swim. The ones that roll together seal best but add bulk around your waist. Drypants are more popular with whitewater boaters than open-water paddlers.
3) A drytop has latex gaskets at the wrist and neck. A semi-drytop does not have a latex neck gasket. There are some new neck gasket materials and designs coming on the market, so the distinction is getting less clear.
4) A double tunnel is just that – two concentric layers of fabric on the lower torso. One goes inside the sprayskirt tunnel, and one goes outside. The outer one usually has velcro & neoprene to help seal to the skirt.
As above and the meanings of the terms are absolutely literal. Splash tops will keep you dry against splashed water but not immersion, drywear is intended to keep you dry in cawse of immersion. Semi-dry clothing will leave you partially wet, drysuits protect you by keeping you dry and wetsuits protect you by keeping you warm when wet. Would that most terms could be interpreted in such a straightforward fashion.
Semi-dry tops are characterized by lighter weight neoprene gaskets or just stretch and velcro closures, drytops have latex or a tighter heavy duty neoprene in some manufactureres.
As above, two piece “dry” systems often aren’t in practice.
Hydroskin is designed to be equivalent to a certain thickness of a traditional neo wetsuit.
The drytop and wetsuit combo (though at 50 degree water I’d add a layer of neo underneath) functions because the wetsuit handles the bit that the bottom of a drytop, unmated will not at all keep out water should you take a swim. The latex gaskets typical at the neck and wrists will only function to keep you dry if you roll up, ie stay in the boat, and have a good tight double tunnel mating with the waist. The latter is why a lot of more advanced paddlers prefer a neo tunnel on their skirt - the usual velcro closures are not as good at keeping out water during a roll.