The point of the example is to provoke a concrete discussion. Appeals to unimaginable are not very useful. The underwater environment doesn't seem as conductive to technological visible-from-a-distance civilization, in any case. Dramatically different civilizations may not go into space, and if we are discussing civilizations whose apparent absence in the sky is suspicious, those recreate a huge chunk of our own civilization.

edit: let's put it this way. The only data point we got on things such as civilization not becoming stagnant, or civilization becoming visible, is our own. The further we go from there the less reason we have to expect those to occur.