You catch it in the Great Marsh in Pastoria City. Go to Area 2 and use a rod (I used a super Rod) in the water in Area 2. If you don't get it the first time, don't worry, keep trying :)
Btw it won't be a very high level, mine was lvl 16.
CARVANHA evolves into SHARPEDO at level 30 (CATCH CARVANHA AT SPRING PATH AND GREAT MASRH)
Pacifalog Town
It is in Route 119
a pokewalker route.
near the weather institute you go past the bridge and go down fish with a good rod or super rod super rod has 100% chance to catch a carvanha
You can find Carvanha in Route 119, use your Super Rod. If you want Sharpedo (final form of Carvanha), you can find it near Mossdeep City, use your Super Rod.
You can use a good rod or super rod to find carvanha on the route southwest from fortree city.
pastoria gret marsh super rod
To get Carvanha: Go to Pastoria Marsh Find some water in any of the 6 areas Use the super rod to fish There is a 35% chance that you will catch a Carvanha.
In Pokémon XD: Gale of Darkness, Carvanha evolves into Sharpedo at level 30. You can catch Carvanha in the game, and training it to reach this level will trigger its evolution. Remember that in this game, Pokémon can also be purified, which is a different mechanic from leveling up.
First of all it depends on what route your fishing in but you catch these pokemon: Magikarp, Staryu, Carvanha, Horsea, Psyduck, Goldeen.
If male around level 24-34 and female 26-33, (CATCH AT SPRING PATH)