Ditto above plus: Check the fit of your main hatch boards, ours are tight on the top to one side of the sliding hatch and loose on the other, if the rain is blowing around we will take water through that gap. Second, check the sliding hatch itself. Not sure what you have on the 28, we have two "U" shaped pieces of nylon attached to the hatch that the hatch slides back and forth on, the starboard one was cracked in a couple of places and pieces were missing when we bought the boat. It did not provide a good slide or a good seal. Third, on our boat the track that the hatch boards sit in has two oblong holes at the bottom for the water to drain into the cockpit, they can get clogged and overflow inside. Lastly, if all else fails, try the old chalk trick. Draw lines around your puddle, when it rains the source of the water running into the puddle will wash away a part of the chalk line (unless it's dripping straight down).
PS. I think the hose idea is a good one, get someone to hose the boat down with you down below.
