Items found at the Lighthouse: Deathblower 1, Deathblower 2, Deathblower 3, G-Godfather, Survival Tent, Z-Charger
Bring Emeralda with you and head to the Lighthouse (it's located at the NorthWest tip of a big island in the Aquvy area). Head North taking the elevator down, then head East until you see a small entrance North. Get out of the gears and go inside, then check the walls to find a Deathblower 1 (at the second floor) and information about HyperMode, Eldridge and Big Joe's past (...What!?). Exit, head South and enter the small East entrance. Get the Survival Tent, save if you want, then exit. Continue South, and from this street you can turn West to get a Deathblower 3 or East at the end of the street to get a Deathblower 2. Head back North to the place where you can save, turn East, then head South inside a small door to find a G-Godfather gun, then head back North and enter another small door to find Big Joe. The treasure chests will give random items and come back if you exit and re-enter the room, but what really matters is Joe's Shop. Some of his gear equipment might be too expensive (And it's ok if you don't want to risk using engines with high output, but low fuel), but upgrade at least your armors. And if you're high on cash, why not get a couple of drives? Anyway, exit the shop when you're done and head East until you reach a wide parking lot. Search for a manhole to the SouthWest and go down.
Head West twice and on the third time, go inside the subway car to get a Z Charger, head up the stairs and move along until you watch some scenes. After some sexual comments by Emeralda (Well, not really), she'll grow up and improve her status significantly. Exit the Lighthouse, and if you have the Mermaid Tear, go back to Shevat and talk to the woman at end of a stairway to get (don't forget to bring Emeralda) a Mermaid Ring. From this point, you can...
Finish the other side quest, the Duneman Isle, or...
Head to the last dungeon, Deus.