Catching Carvanha with Speed Boost?

I just started this game few days ago and im loving it.
But im trying to catch a Carvanha with the skill Speed boost, im using a good rod in route 12. I have like 20 carvanha and 0 with speed boost. Im doing something wrong?
Also, i have de dexnav, but still havent use it, but is possible to use it fishing?