Clearly I was looking for a leak in something, but what? The hard part is figuring out what the plumbers' putty is for. Plumbers' putty is like plasticene -- it isn't very sticky but you can mold it into any shape.
I was looking for the leak in my fiberglass sailboat, along the seam where the decking meets the hull. I removed the drain plug and put the hose from the pump into the hole, sealing around the hose with the putty. Then I squirted the organic oil soap along the seam, pumped air into the compartment, looked for the bubbles, and circled the leak with a pastel. (It has to be some kind of oily marker to stick to soapy fiberglass.)
Once I found the leak I washed the soap off with the bucket. (It has to be organic soap because I was rinsing it off directly into Puget Sound.) I sanded down the leaky seam with the sandpaper and spread the missing item -- marine-grade flexible epoxy sealant -- with the popsicle stick.
Hopefully I got all the leaks. I haven't had a chance to float test it yet...