Welcome to the forum!
Your DS should be pre 1971 and has the single hull with separate seat tanks glued on. Normally those seat tanks were filled with styrofoam blocks (the old kind, not what they use for builders foam now). That foam tends to get waterlogged. Even if it wasn't, your experiments would have made it so. Unless some previous owner removed the foam, you'll need to replace it (with pool noodles for example). You'll find descriptions for that process either here or in the DS1 section of the forum, like this
recent post on foam removal.
The standard method for finding leaks works like you described, except adding food coloring
for better visibility, but I tend to doubt that you will find any leaks this way. Your implied assumption is that the leak in the hull is close to or below the waterline in an area covered by the seat tank. Unless there's a visible crack in the hull on the outside, I'm not encouraged that you'll ever find anything there. You don't report such visible cracks.
If my boat had a crack between seat an hull, it would easily be possible for water to get inside the seat tank from the cockpit during sailing, and then to weep out afterwards. Sources of water in the cockpit include a dripping seal on the CB handle or spray. You don't report whether yours has the original wooden floor gratings still in place. They can hide a small "lake" that sloshes around the boat, esp. when heeled.
Sometimes, the hull-deck joint is not sealed well any more and if you sail your boat vigorously, you can get water in that way. If your seats have cracks along their top edge, where they connect to the hull, water could get into them from there, on the inside.
There's also a chance that your hull has a crack that is well above the waterline. Your method wouldn't find a source of that unless you filled your boat to the level of the coamings with water, something that would most likely cause damage if you try this on land . . .
You don't write whether you keep your boat afloat or sail it dry. If kept on a mooring, another source of water may be rain water. If your seats have the original foam, that water may come out with a delay.
If after due consideration, you still expect water to come in through the hull, go push on the hull on the port side (also shift it on the trailer to be able to see everything). If the hull flexes, there may be a crack that "opens" when flexed. But you should be able to find that by inspection. If the hull is really soft, you may need to take the seat off with a Dremel or similar tool to strengthen the laminate from the inside and then glass it back in.
If you still have original foam, replace it. While you have it out, inspect the inside of the hull through the access port (e.g. by sticking in a camera on a stick and taking close-ups). Then fix the crack along the hull. Test sail the boat before filling with pool noodles and see whether water puddles on the inside.
Let is know whether any of these suggestions make sense and/or fill in some detail.