From http://www.epicurious.com/recipes/food/views/Thai-Spiced-Waterme...

For soup
5 cups coarsely chopped seeded watermelon (from a 4-lb piece, rind discarded)
1 fresh lemongrass stalk*
3 tablespoons finely chopped shallot
1 1/2 tablespoons finely chopped peeled fresh ginger
1 tablespoon finely chopped garlic
1 1/2 tablespoons mild olive oil
1 small hot green chile such as Thai or serrano, finely chopped (including seeds), or to taste
2 tablespoons fresh lime juice, or to taste
3/4 teaspoon salt, or to taste

For crab
10 oz jumbo lump crabmeat (2 cups), picked over
1/4 cup finely chopped fresh cilantro
1 1/2 tablespoons mild olive oil
1/4 teaspoon salt, or to taste
1 T fresh lime juice

Accompaniment: lime wedges

Make soup:

Purée watermelon in a blender until smooth and transfer to a bowl. (Don't wash blender.)

Discard 1 or 2 outer leaves of lemongrass and trim root end. Thinly slice lower 5 to 6 inches of stalk and then mince, discarding remainder.

Cook lemongrass, shallot, ginger, and garlic in oil in a 2-quart heavy saucepan over moderately low heat, stirring, until aromatics are pale golden, about 5 minutes. Add about one third of watermelon purée and simmer over moderate heat, stirring, 5 minutes.

Remove watermelon mixture from heat, then transfer to blender along with chile, lime juice, and salt and blend until smooth (use caution when blending hot liquids).

Add remaining watermelon purée and blend briefly. Season soup with more chile, lime juice, and salt if desired, blending if necessary. Pour soup through a sieve into a bowl, pressing on and then discarding any solids. Chill soup, uncovered, about 2 hours.

Prepare crab:

Toss crabmeat with cilantro, oil, lime and salt.

Divide crab among 4 soup plates, mounding in center, and pour chilled soup around it.