Captain needs to move. If he doesn't want to leave his spot, surfacing is the only option.
This is not correct! As explained in the topic “Activating Silence question” (in the BBG rules forum) there is a possibility to “move zero spaces” by activating the Silence gauge:
I just checked and now I understand why the confusion. There is actually an imprecision in the english rules. The french rules clearly say that the silence allows for 0 to 4 movements and that the captain indicates to his teammates ONE direction.
This will be rewritten in the english rules.