Following the 1527 acquisition of Rijeka and Senj harbours in nowadays' Croatia, the Habsburg dynasty finally gained a much desired exit towards the Adriatic.
However, despite the Reichssturmfahne or "Imperial War Flag" was well established for use on land since the 14th century, no Emperor took care of establishing a naval ensign (for either sea or the Danube rivers) until Charles VI (1685-1740), who ordered the first Imperial warships to be built for use at sea, both in Croatia and Naples.
In those first decades, Imperial ensigns design could vary from one ship to another - bordered with black triangles all around or not, showing a plain yellow field or a striped yellow-black one... It was in 1749 when Empress Maria Theresa established an ultimate design, yellow with a crowned double-headed eagle.
Sources: «Flags of The World» (FOTW) website, Austro-Hungarian Empire: The Habsburg Empire until 1749
Sizes: ensign W12 x L18 mm, mast flags W8 x L12 mm, jack W6 x L9 mm, pennant W4 x L48 mm.