Turn on LED when an electrode or contact is submerged in saltwater
How about just using float on a rod.  When the water rises it can trip a mechanical switch that's been sealed against the elements.  Maybe a commercial one or just build one out of a spring.   Or you could have a little IR sensor also sealed up that when it reflects off the float it sends you your signal.



Here's some waterproof switches I'm sure there are others:


http://www.cpi-nj.com/products/waterproof-switches/limit-switches
http://www.ottoexcellence.com/store/item.aspx?ItemId=28


You could make a simple switch out of two strips of metal, when the two strips of metal are in the water they'd conduct.  You could hide them in a PVC pipe with a capped top and open bottom to keep out the rain.  That would corrode over time though.

You  might also try a hydrostatic pressure sensor like this guy: https://www.sparkfun.com/products/10221



There are other more expensive probably more reliable ones out there as well.  The hydrostatic part of it will keep it from going off in the rain.   It says it holds up in salt water.