Underwater in SH3 was set at 8 meters while Sh5 seems to have set at 6.5.. The exhaust on type VII is not down by the waterline, but high behind just under the flak gun.
This is a game bug in SH5 and it makes decks awash almost useless.

Perhaps the TDW can patch this in his next version of the patcher? I will have to wait until I'm out this storm I'm in to test decks awash again, but I'm pretty sure I can still use binoculars at 7 meters.