The light blue stuff you're seeing are the white bubbles caused by the churn of the ships' propellers.
Water is blue; it absorbs other wavelengths of light to a greater extent than it absorbs blue wavelengths. If you place a long tank of water between you and a white light source, the light source will appear blue. The longer the tank, the more water there is between you and the light, thus the greater the absorption of light and the darker & bluer the water appears.
The water around the ships appears very dark blue because the light from the sky travels deep into the water and is mostly absorbed, leaving little to reflect back into the camera. Whereas the light hitting the white bubbles is mostly reflected into the camera after passing through mere metres of water; however, a little is absorbed. Thus the bubbles appear light blue instead of white.
If the ships were not causing bubbles but dragging bunches of white ribbons a few metres under the surface behind them, those ribbons would appear a similar light blue.