Historically, there was (and is) the Seebataillon, which was originally formed in 1852 and, through various incarnation, fulfilled Naval Infantry tasks throughout the Royal Prussian, Imperial German, and the Third Reich (following a hiatus in the Weimar Republic and various changes after 1935, if I am not mistaken).
Generally, one would use a combination of terms that designates the unit's scope and purpose. Often, this would include a unit number.
A possible one would be:
Marinebataillon Abteilung 123 z.b.V. (z.b.V. abbrev. "zur besonderen Verwendung") which could be translated as "Naval Batallion, Detachment 123 (Special Operations)".
If you could provide some ideas as to size/organisational unit, place of station etc., we could work out something more specific for your purpose.
The current Bundesmarine equivalent is the Kommando Marine Spezialkräfte - roughly, "Special Forces Command (Naval)".