That didn't end well. Pre-war Scapa Flow is not a good place to dock.

[Fahnenbohn]

Originally Posted by Von Due

No, no, no, nuuuuuu. Call it... a friendly visit, to marvel British defense systems and ship design.

[Von Due]

Coming to think of it, visiting these ports in peace time for intelligence gathering isn't that historically inaccurate. Scapa Flow is a little questionable though but in general, ever since the advent of intelligence agencies, pretty much every European nation had a pretty good idea what the other nations' naval capabilities were, how their ports were laid out, how to get in and out, defences etc. Spies and agents from all nations racing across the map all days a week, ships logs being scrutinized by intel, shipyards being under the looking glass, ship registers from insurance companies being read by other nations and so on.

One could think of this friendly visiting business as a way to add some of that into the game.

EDIT
Somewhat to the point albeit not really naval related.
I have a reprint of a German made map of the area around the city of Bergen, Norway. It has some nice little details like the words "Not for the public" (loosely translated) printed on it. On this map, of exquisite detail, you have all the main roads, all the secondary roads, paths, trails, etc in the area of that time, named and some of them have additional details like "suitable for heavy vehicles during winter" or "not suitable for heavy vehicles during spring". The original, to my knowledge was printed before the war. All those little details a traveling tourist could bring back home.

Speaking of tourists. Some years back, a Russian tourist was questioned by the police after being seen around the local airbase here. The airbase is located near a large lake, popular for fishing and canooing and suchlike. He was fishing. With the most state of the art fishing rod Moscow could produce. It had a socket for earplugs on it and he was presumably listening for fish... or radio traffic, possibly.