Flood sensor

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Practically anything is possible, it depends on time, effort and money your prepared to put in. A basic water detector either by floating switch in a sump/container or water shorting two wires via a detector circuit is the obvious way to go.

The fundamental problem will be weather proofing against rain and other stuff like bugs, crawlies, plants/trees/weeds, next door's cat. For me it would seem not an electronics problem as such, more of a mechanical one which would differentiate sea water and everything else by clever placement of your widget.

Pesumably we are talking some form of Ph sensor to detect alkilinity of sea water as opposed to (acid) rainwater.

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Anything buried in the garden and exposed to the elements will need routine maintenance and inspection for just this reason. I would be interested to see a solution.

Graham