I've just tried placing a simple model under the water, and it worked with some problems.
First you have to place the model in sketchup well under the coordinates of the map you get from GE, it seems that the highest point in any underwater topography you import will always be 0 in sketchup.
And to view the model in GE from an underwater perspective you have to turn the water surface off, so no cool pictures from below the seas for the sketchup fans, wich is disapointing.
water 3.jpg
Water 1.jpg
water 2.jpg