I compiled my map without my custom dem-poly option and the result is as I tried to explained in my first message. You can see the result at :

Artifacts are also still present along shorelines. But it seems to be normal : I checked SRTM1 tiles and the problem is in source data. In fact, mkgmap works nicely !

Thank you for your work and also many thanks to Andzej Popowski and Frank Stinner.