Faster: Above or below water?

Depends on the swimmer, but there's a reason Phelps comes off the wall and kicks for so long.   Other guys have 4 or 5 strokes before he pops up.  Can't keep that up for 50 meters though

From the above post, unless you truly mean "above" the water, otherwise, you're still "in" in the water.  You can be more streamlined under.

That's disputed by the fastest swimmers in the world who, clearly, swim faster underwater. Recovering your stroke underwater is slower, but when you are dolphin kicking in a streamlined position, there's no stroke recovery (this is, in part, why breaststroke is slowest).

Being streamlined is fastest.

When you break the surface, you break streamline, and introduce air to your paddles.

Yep...15m. DQ if you go past. And that's exactly why they changed the rule--cause it's faster.

Misty Hyman, who upset Susie O'Neil in Athens in teh 200 fly, used to dolphin kick almost the entire first 50 meters of that race. Likewise Dave Berkhoff in teh backstroke events.

Under.  Definitely under!

Surface drag (from the seperation boundry of water/air) and interference (trailing vorticies/cavitation) drag from swimming on the surface are signifigantly higher than form drag (body shape and streamlined) under water.  You experience considerably less drag when you are totally submerged.  A submarine can travel at speeds almost 5 times faster under water than they can on the surface.

When Phelps gives a massive push off the wall, stays and dolphin kicks under water for 10 yards farther than the competition he is going much faster and passing them.  The longer you can stay under, the faster you'll be.