Why is it that in the pilot episode the Aquaphibians spoke in the 'gargle' language, but in this episode they spoke in English (with a bubble effect?)
In addition, Marina must be a fast swimmer - she catches up with Stingray after they've left Pacifica! Since Stingray travels at 6000 knots, to catch up with them would mean travelling fast!

She and Oink swim like torpedoes in The Golden Sea

Maybe you find different races of Aquaphibians, from different areas - like humans. So some speak English and some speak "Aquaphibian".
Maybe Marina caught up because Troy was sulking and hoping she would come back!

Testing Marina's loyalty seemed an awful lot like the old tests of witch-ducking. "If she dies, she's innocent, if she lives, she's guilty and will have to be executed...."

Were those vapors coming from the plant only supposed to be visible to the viewer? Or were Atlanta and Marina incredibly blind?

Troy is really slow on the uptake. Marina has been Titan's slave for who knows how long, Troy brings her directly to Marineville, and he has a hard time figuring out that just maybe she might possibly want to go home for a visit.

In point of fact, that's the first thing they should have tried to find out about Marina. And it would have been courteous to send word to Pacifica that one of its citizens had been freed from slavery. (Of course, the underwater grapevine seems to work pretty well.)

That plant seems to be pretty smart. It only starts sucking air when there is a person in the room. Notice that it allowed Atlanta's quarters to fill back up with air.

BTW, just how did they get Marina into the apartment, anyway? It must have seemed odd to her. "Oh, Father and I won't be there, but just go on in and make yourself at home--play the piano if you like."