You'll probably need to fly really close to the waves to avoid detection. It seems there was a second sysetem called Chain Home LOW that was used to detect aircraft as low as 500 ft (150m):
http://en.wikipedia.org/wiki/Chain_Home
http://en.wikipedia.org/wiki/Chain_Home_Low
Note (later in the war):
"Later in World War II, centimetric Chain Home Extra Low (CHEL) radar were often co-sited with CHL sites. As the name implies, these sets provided radar coverage at even lower altitudes than was possible with Chain Home Low."
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