Thanks.
I found two very nice photos showing Navy soldiers with K98 in forums over here.
One from I. Seeflieger-Abteilung, the other one from 2. Matrosen Artillerie-Regiment. Both photos are dated 1917.
A fellow collector told me that the Navy units on the Western front had about 5,000 K98. Including naval airship and air force units.
4,016 K98 with infantry and artillery units (unknown date)
107 K98 with the Marine-Luftschiff-Detachement and I. Marine-Fliegerabteilung (October 1915)
810 K98 with B.M.A. Liebau und I. Seefliegerabteilung (October 1915)
If I can find out more, I'll let you know.