The French navy can't match the British navy alone. The British have more ships, better ships, and better seamen. An alliance between the French navy with the navies of Spain, Holland, and the Scandinavian countries, which all have excellent seamen that the French lack, could be more than enough to defeat the RN. France should simply point out to these maritime countries that Britain has repeatedly violated the freedom of the seas, that it is needed to dismantle the British navy to ensure this freedom. That would require Napoleon to be skilled in diplomacy as he is in generalship, though.
Here's an interesting What If and of course as were all talking Alternate
History on the forum, its well within the realms of possibility.
I'm sure many members are aware of the Robert Fulton submarine,
the Nautilius that was first test trialed in dives on the Seine at Rouen,
in the Saint-Gervais dock from the 29th July 1800.
The trials, sponsored by the French Minister of Marine, was to prove
the capabilities of the Nautilius as a weapon of war ( or secret weapon )
against the Royal Naval blockade of French ports during the French
Revolutionary & Napoleonic Wars.
Wikipedia quote:
On July 3, 1801 at Le Havre, Fulton took down the revised Nautilus to
the then remarkable depth of 25 feet (7.6 m). With his three crewmen
and two candles burning he remained for an hour without difficulty.
Adding a copper "bomb" (globe) containing 200 ft3 (5.7m3) of air
extended the time underwater for the crew for at least four and a
half hours. However, one of the renovations included
a 1.5 in (38 mm) diameter glass in the dome, whose light he found
sufficient for reading a watch, making candles during daylight activities
unnecessary.
Speed trials put Nautilus at two knots on the surface, and covering
400m in 7 min.
He also discovered that compasses worked underwater exactly as
on land.
The first trial of a carcass destroyed a 40-foot sloop provided by the
Admiralty. Fulton suggested that not only should they be used against
specific ships by submarines, but be set floating into harbors and into
estuaries with the tide to wreak havoc at random.
The overseeing committee enthusiastically recommended the building
of two brass subs, 36 ft (11 m) long, 12 ft (3.7 m) wide, with a crew
of eight, and air for eight hours of submersion.
In September, Napoleon expressed interest in seeing the Nautilus,
only to find that, as it had leaked badly, Fulton had her dismantled and
the more important bits destroyed at the end of the tests.
Despite the many reports of success by reliable witnesses like the
Prefect Marine of Brest, Napoleon decided Fulton was a swindler and
charlatan.
The French navy had no enthusiasm for a weapon they preferred to
think suicidal for the crews (though Fulton had no problems).
Certainly, it would been overwhelmingly destructive for conventional
ships.
Of course the big 'what if' in all this is, what would have happened
if the September trials had proved a big success with Napoleon.
Who would have realised the potential of the submarines 'underwater
stealth capabilities' in sneaking up against the royal navies, Ships of
the Line and blowing them up, out of the water.
Obviously the French wouldn't need to build many. Just enough for
service amongst the French Naval ports of Brest, La Harve and
Toulon. For use against the Royal Navies blockade squadrons.
This would have added a new dimension to the Napoleonic Wars
being a constant threat to the Royal Navies ability, in keeping
the French Navy bottled up in their home ports.