http://www.fleetairarmarchive.net/Aircraft/Walrus.htm
Gunners have a good all-around view.
As to viewing distance, a non-radar-equiped plane might spot a wake at 20 km or more, if he's high enough. Of course, the boat's crew could see him more easily too. The real danger came from the mid-war radar-equiped Liberators. They had extremely long range and a good radar that could pick up a conning tower from a looooong way out.