Where did Marina come from?

[HappyPumpkin]

Her name.

The japanese version,and the dubs,follows the "First default name is their name" pattern for main characters without a NPC name.

Red = Ash/Satoshi.
Green = Gary/Shigeru
Pearl = Barry/Jun.

In Jimmy/Kenta's case,a Youngster from Gold/Silver named "Kenta" was renamed "Jimmy" in the english version.

But,Marina? Where does that come from?

It should be Amanda,and Chiho.

Why did they change it?

 

[Doctor Oak]

Because they did.

 

[Ampharos King]

Marina because of her blue hair and the fact she uses a Croconaw.
Although yeah, we had a filler character with the same name who was a water-type specialist as well.

 

[TTEchidna]

Beats me, Kenta came outta nowhere, too. It's just coincidental that there's a Kenta in GSC, whose English name was Jimmy, so they called him Jimmy. Same with Junichi, there's one of those in GSC, whose English name was Vincent. Marina was Marina in Japan, and since there was no Marina in the games they just decided to use it.

And then named Eusine Eugene. Fail 4Kids, fail. You check RANDOM TRAINERS in GSC but don't know the name of a MAJOR ONE?

 

[Aladar]

As others said because of the blue hair and water starter...and besides Marina is a fine name...