Scaramanga
Brickhead
I don't think any kind of historical statement is trying to be made with the U-Boat, it's just for gameplay/balance purposes. If everyone fielded their best land troop that had one, there would be a lot of double knockouts. Also, Frederick has enough military bonuses himself that with a land UU he could adopt Democracy and still destroy everyone in battle, but now you have to think big in order to leverage a submarine. The U-boat is so specific in its task of breaking up a hostile "Atlantic" trading bloc rather than becoming a naval power that the land power aspect should still shine through.