[Aktungbby]

Well in several of the games from AoD, SH II and SH V the top speed is reduced when awash which would create more resistance; whether the AI fuel consumption is programmed to reflect this 'added drag' is unknown to me. Found this reference to 'decks awash'

... command of Captain Genichi Shibata, was much faster than Agwiworld, the Japanese faced a dilemma. The swells were heavy at the time, and Shibata knew that an attempt to overtake the fleeing American tanker with I-23‘s decks awash would affect his gun’s accuracy and could even result in the loss of some gun crewmen.

http://www.historynet.com/japanese-submarines-prowl-the-us-pacific-coastline-in-1941.htm Some useful propaganda: http://www.ibiblio.org/hyperwar/USN/ref/Lookouts/ "At night, submarines often lay to with their decks awash while charging batteries. If the sea is not too rough, they may cruise with the decks awash. In this condition, a submarine can make a crash dive in a matter of seconds. A submarine awash presents a very small target, and this makes a lookout's job still more difficult. When awash, a submarine often has a long streak of foam both forward and abaft the conning tower. On very dark nights, this foam may be more visible than the conning tower itself."