There is a resistance building in Atlantis. It's not Aquaman, Dolphin, Vulko, or the Widowhood. It is led by the former keeper of the Royal Bestiary: a man named Jurok Byss. He leads the Undercurrent, and their goal is to overthrow King Corum Rath.
The Undercurrent attacks one of the royal armories, but they find themselves severely outgunned. Thankfully, someone has been watching them and supports their cause.
Elsewhere, Mera struggles now that she has found her way into Atlantis, and she runs across a crime lord on the rise.
Aquaman looks to be reaching the climax and conclusion of "Underworld" with the establishment of the Undercurrent and Mera's arrival in Atlantis. This issue, consequently, feels like the calm before the storm. There is action, but it is primarily in the prelude to figuring out what the role of Byss and his Undercurrent will be.
The Mera subplot looks like it will be leading into a future story arc after "Underworld." Likely, she will join up with Arthur, but that crime lord doesn't look likely to come into play before this story is over.
Jurok Byss is a likable character already, though. He's earnest, impulsive, and friendly. This seems like a character who could become a good fixture in the Aquaman side of the DC Universe.
It goes without seeing that the anti-Trump message here is a bit stronger than usual. I would like it more if it got a bit more focused and specific. "Underworld" as a whole has been fairly anti-Trump, and the Undercurrent is the resistance against Trump. It could even be compared to Antifa, though I'm not sure how close that comparison could be made after one appearance.
Stepjan Sejic continues to be Stepjan Sejic. He is an incredible artist, and nearly every panel is a masterpiece. Admittedly, some look the slightest bit rough now and then, but that is only because it is comparing Sejic against Sejic. Slightly underdone Sejic is still better than many artists.
While this is an unexciting issue of Aquaman to discuss, it is still another solid entry in Dan Abnett and Sejic's tenure on Aquaman. It lays the groundwork for the finale pretty well, and next issue should be quite something. I do still recommend this issue, and you should definitely read it.