I decided to add a few elements from the show into it. Just because.
It's still mostly based on the games, however.

It only takes five minutes to catch up to it (thank God it floats!), but by the time I get back to shore my clothes are dripping wet,
and Gastly still isn't done laughing.

"Yeah, yeah, laugh it up gas ball." I grumble as I pass him. I look down at the ball in my hands to see it looking perfectly normal,
but I'm a bit leery about using it all the same. Is it water proof?

If it's not, then how would anyone catch water type pokemon?

I pop it open and a little pool of liquid drips onto the beach. What if I forced Gastly into it, only to have it short out?

That's a question I don't particularly want to have an answer to. With a sigh I snap the ball shut again and slip it into my bag.

Gastly's insane laughing fit is almost done by this point, and it looks like he just has some leftover giggles to get past. Gastly has an incredible evil laugh, by the way. If he really tries, he sounds exactly like the picturesque movie villain.
I just have to wonder if it comes with the territory of being a ghost type, or if he had to work at it.

"Well, come on Gastly. I don't think we'll be putting you in your ball anytime soon. Just float right behind me, okay? No killing things."

I'm waiting for a response until I start walking, just in case. I could easily see him just wandering off somewhere.

Still no response. "Gastly, what are you doing back there- AAAH! NO! STOP THAT!"

Gastly has a wicked grin on his face as he engulfs some poor random pokemon with blue gas. I dash towards him,
and only when I get close enough to have the smog touch me as well does he back off.

With the gas cleared, the traumatized thing is revealed to be a bird-like pokemon, black and white in color and not even up to my knee. That reminds me, I probably need a pokedex, but that's not likely to happen very soon.

"Gastly, was it really necessary to do that? That thing was way below your skill level."

Gastly quickly gestured down to it before looking back up at me, "Greer!"

His expression tells me that he actually thinks that was supposed to convince me his actions were righteous.

"You can't simply go around, knocking out bird pokemon for no reason! Just because it has wings-"

"Starly!"

Startled, I turn around to see an Officer Jenny holding the fainted pokemon to her chest.
She turns to me after returning the 'starly', and raises another Poke Ball.

I think my jaw dropped sometime during all this. Out of the corner of my eye, I can see Gastly wearing the smuggest damn smile I've ever seen. It practically screams, 'Told you so.'

"You! Kid! We're going back to your home right now, your father has been worried sick about you!"

"Uhhh... How about no?" I call back, my eyes darting every which way, looking for any other cops.

Having given a warning, Officer Jenny throws the Poke Ball she's been holding, "Growlithe, go!" As she lets the Poke Ball fly, she swiftly raises her handheld radio and barks a few commands into it. I only caught three words, found the runaway, by the time the Poke Ball pops and a... thing comes out. Wait, did that thing just spit fire?

That radio probably spells trouble. I wonder if we can get out of here before the officer's backup shows.

I turn around to see Gastly looking expectantly at me.

"You know what would be great right about now? For you to do as I say."

Gastly gives me a patronizing smile, the kind your parents give you when telling you that fire burns.

"... Or you could just ignore me and do your thing."

Great, I think as Gastly flies by me towards the fire-dog-thing. Just great. My first real pokemon battle, and I don't get to do anything.