My Goop didn't harden

If you can't find marine goop plumbers goop works fine. I think they're all pretty much the same formula.

Yup, I believe Marine Goop has UV stabilizer, but inside the hull it shouldn't see much sunshine.

The marine goop on my puck dried and cured overnight just fine, with my hatches wide open. I do all my gooping and epoxying, etc out in the garage/shop.  It took several days for it to quit giving off vapors, though. Five days or so later I could still smell the vapors if I closed the hatches for a while and then re-opened them. Now, I can't smell it anymore when I open the hatches.

I like that 3M 4200 for sealing around thru-hull fittings and screws. So far I haven't noticed any problems with it curing. But if it gets put on too thick, I think it might take a long time for all of it to cure. Sounds like if you get it on thick enough, you might get a permanent "liquid center."
I had put the extension applicator tip on the tube and left it on, plugged with a screw, and the 4200 only cured back about 3/4" into the tip. I store my 4200 in a ziplock bag in my fridge.

 I found plumbers goop at lowes. Ill try that and now switch over to the water method too.

Well my foam didn't hold the water. No matter though as the transducer seemed to work just fine.

Saw the title of this thread (My Goop didn't harden) and the first thing I thought of was,
 There's Something About Mary