After having found the first treasure, and having to find the remaining maps and treasures on my own, I've managed to find maps 3 and 8, both with negative coordinates. The problem is that whether I go west or east, I end up coming to a wall I can't possibly climb. Besides the parrot, I only have the shovel and throwing hook.

Will someone please point me in the right direction?

You should go to the east. The westmost point can only be unlocked by coming from the other side, because the world of TAG is round. More information (like a screenshot at your eastmost place) would be nice.

Also, go inside Brooks and Barney's House on Cargale Island and fall into the cellar by going under the table (or unlock the cellar door with a lockpick). Use a throwing hook on the lever in order to get Map 2. Then explore AngelOak Island further for 2nd Treasure.

First off, I remembered it as being a wall, but it's actually the waterfall at 1109,0 I meant.

Your advice did help though. I was expecting to get an upgrade to be able to throw the hook upwards, so it never occured to me that I had to jump and throw it at the same time instead. I also had gotten stuck in the tree previously, having found what I could've sworn was a small mushroom, right before the room with the floating platforms, but when I went back it was a big mushroom, so I could continue to the top and get the 2nd Treasure as you said.

However, this has now left me back at square 1, with the exact same problem. I believe I've explored all I can explore, and I know of a few places where I need items that I just haven't gotten yet, so I'm at a loss of what to do next. Unfortunately the 2nd Treasure Map only gave me treasure, but no new items.

The waterfall? Sounds like you're stuck on Boracay Island.

On the west side, you should be able to jump on one of the flowers. From there, jump on the tree branches, then on the vines and more branches. Get the dimensional fragment and watch out for the monkeys. After passing the first waterfall, swim under the second one and you'll find a secret cave with sand. Dig there (somewhre on the left side) and you'll find a cannon. Jump on more vines and you'll be able to go further.

Also, you can go back to Zwann Island with the cannon and shoot a rope holding the switch. After activating it, you can go up and buy a flag for your boat.

You were right, but the key thing here were the vines. I did not know they could be used to latch on so I could jump between them and use them as platforms. I got all the way to Darausel, the city of GloboCorp now and got a treasure map there, so hopefully everything will be smooth sailing from here on out, at least I don't feel stuck up a creek without a paddle anymore.

Thanks for your help.