That gas line project is a major one, and not without controversies over here.
That they so carefully looking at the sea bed is not so much for preservation of antiques as for fear of encountering WW II remnants of explosives (Mainly mines) though.
That old shipwrecks were found is in itself not surprising - after all the Baltic Sea is practically an inland sea with mercantile traffic dating back to the Vikings. Probably the worldwide most and oldest 'international' commercial traffic outside the Mediterranean.
The controversies btw are less environmental as political/economic, since the line will circumvent the eastern European states (mainly the Ukraine, the Baltic states and Poland) through which the current landbased lines pass. It will as such allow direct delivery to Germany, and as a side effect allow the Russians to cut the Ukraine from gas delivery in the case they should (again) argue about the price. Currently such disputes resulted in immediate reaction from Germany, as they couldn't cut off the line while the Ukrainians had used the passing gas destined for Germany for their own needs.
(The land line(s) were build using German technology with the Mannesmann patent on seamless pipes - one of the major successes of German technology export industry)