I would try a lot of boiling water and a gentle tap... maybe put the steel ring up to it and tap that. What exactly is it stick on? Surely not the shaft as it would act as a shaft brake
Circumstances not explained, but has the seized seal been found at relaunch time? After leaving mine over winter I found that the stainless face had suffered corrosion, probably exacerbated by chlorides. Ever since then I flush with fresh water at haul out and the problem has not recurred.