England
I consider France to be the strongest country in gunboat. France enjoys two advantages that no one else can claim together:
(1) Great defensive capabilities
(2) Build sites close to the stalemate line
(1) is vital to success in any gunboat game. The countries which can't claim those capabilities must adjust their play to account for their starting vulnerability. Starting off with a good defensive position is like starting the game with part of it already won.
(2) is the key to victory. France and Russia are generally considered the two most powerful countries in Classic Diplomacy (regardless of variant) in large part because of this offensive positioning, which gives them an above-average shot at converting a good midgame position into a solo win.
France, like Italy and Turkey, and to a lesser extent Austria and England, has strong defensive capabilities which give it a good shot at surviving the earlygame and getting into the middlegame with a strong position. France, like Russia and Germany, has excellent building sites, which reduce the deployment time of newly-built units to relevant fronts for trying to convert a good middlegame position into a solo win.
France is the only country that combines both of these features, which makes it the best country in gunboat.
Especially in higher-level play, though, this tends to paint a giant bulls'-eye on France. England and Germany, though wary of each other as potential rivals in Scandinavia, are constantly on the lookout for a strong French start, and Italy, the eternal opportunist, will take any realistic chance to steal the rightful French clay of Marseilles and Iberia. Weakness invites death.
The first priority of any French player in gunboat should be to show the other players that you are up to the task of marshaling France's significant defensive advantages to full capacity.
France can fairly leisurely attack whomever it wants if it can successfully ward off any glancing blows from its neighbors first, as France's defensive position already incentivizes its neighbors to look elsewhere for early expansion: expansion which will likely commit the neighbors to a significant conflict which leaves them open to attack from an uncommitted French player. This fate befalls England pretty frequently, for example, where England's Scandinavian ambitions often get rewarded by French fleets "backdooring" England by moving from Mid-Atlantic Ocean to Irish Sea or North Atlantic Ocean on the exact turn that English ships move into Barents Sea or Skaggerack to press an advantage.
But the capacity to do this depends upon preserving your territorial integrity against any possible challenges in 1901. Good players who roll one of the countries neighboring France are acutely aware of France's potential to dominate a game. England and Italy, in particular, can secure all of their "reasonable" 1901 SC targets without committing all of their units, and it's pretty common to see at least one of them use the leftover unit to harass France by occupying border provinces (English Channel and Piedmont) and threatening attacks on France's home centers. Germany rarely tries for Burgundy in S1901, because it has a stake in too many neutrals and because of a certain meta trend (see below), but a foreign unit entering Burgundy is substantially more disastrous than one entering the Channel or Piedmont, so care should be taken against that possibility as well.
In higher-level play, by far the most common opening is the Maginot Defense:
F Brest -> Mid-Atlantic Ocean
A Paris -> Burgundy
A Marseilles S A Paris -> Burgundy
This opening checks off a couple of boxes:
- If no one has threatened you in S1901, you're adjacent to both your Iberian centers and Belgium
- Adventurous German players who try to move to Burgundy in S1901 get maximally punished for it
- Any Italian move to Piedmont can be answered by covering Marseilles with Burgundy without deferring the capture of either Iberian center
This opening is weak to English Channel openings from England, though. It's rather difficult for France to move the fleet in Mid-Atlantic Ocean back to cover Brest in A1901, because if England doesn't try to capture Brest in A1901, France loses a lot of tempo relative to England; France will effectively have "wasted" the entire first year with that fleet, while England will get two moves to try to obtain an advantageous position. Also, covering Brest is pretty disastrous if it means France has to build a fleet in Marseilles (or possibly not at all, if France also had to cover Marseilles). I don't see a great fix to this from this opening, but I'm also not sure what the alternatives are.
I'll open the floor here. Are there any interesting openings you like as France in gunboat? What do you like to do about the possibilities of attack from your neighbors? How do you like to approach Belgium?