The hunter use 'Storm sea' card on the sea to force Dracula's disembarking. Can Dracula's encounter mature ?

You mean because Dracula being forced to land will add a location card and potentially force the last card off the track. Yes, I would say you can mature that token. The token doesn't care why it's being forced off the end of the track, only that it is.

Sure. When Dracula is on the sea, he could mature encounters. When he disembark, Dracula put the location card in the trail, and the card which is on the extreme left exit the trail of Dracula could mature the encounter it has.