There is more to consider than line of sight...
Fresnel effects will cut the signal down as well.
See
  http://www.winncom.com/glossary.aspx?term=23
http://www.connect802.com/wcu/2005/newsletter_050501.htm

Comments on 2.4 GHz through water
http://expertanswercenter.techtarget.com/eac/knowledgebaseAnswer/ 
0,295199,sid63_gci976435,00.html

Regards

AGSC
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