There are several Swimmers on the route for you to fight, but other than that, nothing too eventful. There's a female Swimmer directly north from the starting point in the bay, in a little area that is easily overlooked (hence why I brought her to your attention). She has a Seel L64 and a Alomomola L64 on her team.
I'll just list all of the Trainers on the route rather than worry about their position, because the rest are easy to find and they're all easy to avoid. As for the female Swimmers, one female Swimmer has a Shellder L63, a Chinchou L63, and a Jellicent L63. Another has a Corsola L65. Now, for the male Swimmers... One has two Wailmer L64, the second one has a Quagsire L64 and a Golduck L64. The last one has a Staryu L63, a Jellicent L63, and a Wingull L63.
Aside from the Swimmers, there are also a few dark blue circles in the water. If one of your Pokemon has Dive, you can press A while on one to begin your descent into the water. When you do so, press and hold Up and you'll swim downwards into the depths. You can also press Down to return back. Left and Right don't really take you anywhere, so just swim down to the ruins.
Abyssal Ruins 1F
Here is a map of the Abyssal Ruins 1F. Click the map to enlarge! (Thanks to RLFHOG3 for the map!)
(NOTE: The Relic Bronze is actually "Relic Copper". I did the editing and messed it up when changing from Japanese names to English names, so I'll fix it next time I edit that image.)
Make sure you have a Pokemon with Flash and a Pokemon with Strength! You'll need these three things in order to get through to the later floors.
You'll only have a certain number of steps you can take before you'll be forced back to the surface, so it may take multiple trips in order to explore everything down here. There are tons of items for you to acquire down here, so that's why there's a map to help ensure you get everything successfully! The northern light-blue entrance represents the northern most one, the western one represents the western entrance, etc.
See that white-on-red tile near the center? You're going to want to go there after you've picked up all of the items you want on the bottom floor! Since it will take several trips to get all of the items in this place, it is best to clear out each floor for its valuable loot before trying to proceed to the next floor.
You will have several warnings of the "something is coming!" sort of thing. The first three times you hear shaking or have random dialogue (not counting the forced "switch" when you first enter), it won't be an issue, but the fourth time will result in your ejection from the ruins. You can just go right back in, though.
The majority of items on the first floor are the different elemental plates which can be used both to boost the power of the holder's attacks of that type by 20%, but also to change Arceus' type while it is holding the respective plate. This is the only place in the game you can acquire them, so it is sort of a big deal, although the Splash Plate and Draco Plate are on Route 13 rather than down here. The rest of the items are all items you can sell to a man in Undella Town, over in the big mansion to the west, for a pretty penny.
Once you've gotten all of the items you want, follow the red line you see on the map above. Talk to the purple pillar at the very center of the map (represented on the map by the white-on-red circle) and it will slide aside as long as you've made it to that pillar in 190 steps or less. You can do this by following the line on the map without taking any detours. No dawdling! (It's best you pick up all of the items on separate trips.)
Now swim forward and you'll find a path leading to the second floor.
Abyssal Ruins 2F
Here is a map of the Abyssal Ruins 2F. (Thanks to RLFHOG3 for the map!)
There's a lot more loot waiting for you on the second floor. Thankfully it is a lot less expansive of a floor than the previous one, but it will still likely take you a few trips before you can pick all of it up.
The quickest way to get to the next floor is to follow the red line shown on the map. When you get to the purple pillar blocking your path, have one of your Pokemon use Flash while standing adjacent to the pillar to move it out of the way. You'll then be able to enter the 3rd floor by heading either north or south, each leading to a slightly different entrance (doesn't matter which you take, really).
Abyssal Ruins 3F
Here is a map of the Abyssal Ruins 3F. (Thanks to RLFHOG3 for the map!)
This is the last floor you really need to worry about. It should hopefully take only two trips to collect everything if you've been following the red lines to get here. You'll be able to sell these items for a lot of money, so be sure to pick them all up!
The red line shows you the very simple path to get to the pillar blocking your way to the final floor. As you can see, it's not too difficult; basically just heading due east from your start point, regardless of which entrance you came from.
When you're ready to move on to the final floor, approach the purple pillar marked on the map with the white-on-red circle and then have one of your Pokemon use Strength to force the pillar to move out of the way.
Abyssal Ruins 4F
There's virtually nothing here for you to worry about except one item planted squarely in the middle of the room — directly to your north. It's the Relic Crown, and it's the most valuable item in the game — able to be sold for $300,000! At least if you're looking to sell it, that is. Pick it up and claim victory over the Underwater Ruins!
At this point, just swim around until you're ejected from the ruins, since there's no other exit for you. You can examine some of the other purple pillars in the room if you've managed to decipher them, but it's probably not of importance to you at this point anyway.
All of the treasure you picked up in the Underwater Ruins is worth a lot of money! But you need to make sure you give it to the right person. Inside of Undella Town, there's a mansion off to the west. The Black Belt inside of it is the guy you want to talk to if you're looking to peddle your newly obtained wares to. How much is he willing to pay for these items? Here's a list of his rates!
- Relic Copper (x6): $1,000
- Relic Silver (x6): $5,000
- Relic Gold (x7): $10,000
- Relic Vase (x4): $50,000
- Relic Bracelet (x5): $100,000
- Relic Statue (x3): $200,000
- Relic Crown (x1): $300,000
You can sell all of your loot for a combined price of $1,706,000!! That's a ton of money, but don't worry, because unlike previous games, you can have more than the usual $999,999 in your wallet. Go ahead and sell whatever you'd like of it, or you can always trade it to a friend or another version of yours to give it some extra funds (after they've gotten to Undella Town, of course).