[shatner]

The argument for not being able to cast star/brightness spells underwater has been that there is too much intervening water for the light to reach that far down. That's a reasonable argument aside from the fact that you can still cast Light of the Northern Star underwater. However, it is not consistent with the game overall.

If fighting underwater is so deep that light from the surface can't reach you... then you are already fighting in darkness and the effect should be auto-cast. If light from the surface can reach you underwater, then casting a spell (darkness) which blocks that light should have the effect of plunging the battlefield into darkness. Either way, darkness should work underwater.