Roger, I rec that you get back to basics. Gasket on bottom, plastic sleeve (make sure the notch is aligned with the groove inside thermostat housing), thermostat, then O-ring on top (and sitting in it's groove in the thermostat housing). Torque to 30 ft/lbs.
I believe your gasket provides continuity for the ECT so you can't use a sealant.
2006 Bayliner BR205
A1 Gen 2 1.47
5.0 carb engine