This snippet of text is so ambiguous, you'll need to tread carefully here!

It seems to me likely that this 'level detector switch' is installed so as to turn on an electric bilge pump before the level of the water in the bilge gets too high.

In FR > EN texts I encounter very often, they refer to 'pompe de cale' as 'bilge pump', and 'cale' is the bilge space itself.

As for the sonar bit, I can only assume that this is the bilge compartment where the sonar is installed (hence presumably why they don't want too much water slopping around!). AFAIK, 'sonar' is still 'sonar' in FR.

I find it hard to imagine any other logical technical interpretation; I wouldn't have thought that the sonar per se would possess a 'bilge', nor would I have thought that the level detector switch in the bilge would be there to turn the sonar on. But you do need to be aware of the potential ambiguity of this telegraphic fragment.

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Note added at 2 hrs (2008-05-27 10:26:51 GMT)
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Je comprends parfaitement le souci de Florence, mais que ça soit militaire ou civil, il ne faut pas confondre techniquement le 'sondeur' (= echo sounder, certes, une forme de sonar) avec le sonar, qui est très largement supérieur sur plusieurs plans. Je ne crois pas que l'auteur se serait ainsi trompé...