Initial thoughts.
There’s definitely some stakes and questions here: What do the Atlantians need Leo for? Who wants to stop him from getting there? What’s the deal with Fuko’s outsider nature from other Atlantians (are they all about racial purity? which is a whole bag of bad stuff)? Looking forward to getting those questions answered.
Pages 7-11 have a bit of talking heads syndrome, the same folks talking without a lot of action (which can be dealt with, except they’re on a cramped sub that limits their ability to move around). It also is some exposition dumping which, since Leo is any outsider, makes sense, but it is also a third of the issue. Page 11 is good and if the “ping” on page 9 was supposed to be foreshadowing, I feel like it’s not obvious in hindsight. But that could also be something done in the artwork (having the same SFX style with the ping on page 9 and on page 10).
Unless you were going to have page one on the inside cover, pages 5 and 6 would be on opposite side of the same paper. Usually your full-spread pages go from even page to odd page.
Out of everyone, Trace seems to have the least amount of personality shown and a lot of it seems a bit uneven. This isn’t a big deal since this is only the first issue, but you asked about unique voices and his seems limited to “angry with my dad.” His recruitment scene seems like it could be shorthanded to “wake up loser, we’re going to Atlantis.” And I find it odd that Trace is being coy about having a sub given his family’s history of the Atlantians, given that it’s an Atlantian asking him about it. (I’m assuming that Trace knows Ninjess’s secret, given they’re teammates. Hell I wouldn’t be surprised if he got her the spot on the team to help acclimate her to the surface world if she has an Outsider-style background.)
As for pacing, this is generally how I like to break it down.
Page 1: Introduce Leo and Fuko.
Pages 2-3: “We’re going to Atlantis” and recruiting Trace
Pages 4-5: Introducing the sub and the wonders of the ocean.
Pages 6-7: Wonders continued. Background of Atlantis.
Pages 8-9: Nautilus’s history with Atlantis and foreshadowing the Kraken.
Pages 10-11: More background on Atlantis and figuring out something is wrong.
Page 12: The Kraken revealed.
These are what I see the story of each page being. Page 12 is great. The line with the reveal of the kraken are perfect cliffhanger material to get people to read the next issue. 6-11 are a little slow and I feel like they could be compacted a little more or be beefed up with some flashback panels while Trace is describing his family’s relationship with Atlantis.
There’s also very little visual storytelling going on. It’s definitely one of the hard parts of comics, but I could imagine the “mutant coral part” could have been foreshadowed with a panel of Leo giving the sub an odd look while they we were boarding or something.
Overall, very enjoyable to read with a couple of “new to writing comics” hiccups that I could see being sorted out easily enough. I’d pick up a copy of it.
(Also, not to make anything of it, but I did do thumbnails of the first two pages last night before I went to bed. Just to make sure they flowed okay. That’s totally the only reason. 
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