When in a completely closed-off room where there should be no light, a water source can emit sun light (sl) under a very specific set of conditions: First, there must be solid blocks around the water source in a very specific manner (see attached image). Then, no light must be entering the room from any point except for directly above the water source. Finally, seal off the room by placing a solid block in the only open part directly above the water source (blue wool in the attachments), and then the water source block will emit sunlight until it has a block update or another opaque block is placed directly above the light-emitting water source.

Other notes: Not yet tested on SMP.

Updated to include 1.7.10