14 August 2017 - UPDATE. The fish table for the Trent pool has been reverted back to the Calpheon default and removing the Kamasylvia fish species, including the chance of the 'Message in a Bottle' item. Therefore, the only current way to obtain them is via harpooning. It is no longer possible to fish the bottle item, only via harpooning until Kamasylvia arrives.
16 August 2017 - UPDATE. Now confirmed as a low drop chance from the current Terrmian Waterpark open waters via fishing since the 16 August 2017 Patch Notes, which added a new fish table for the event. Set to run until the 30 August 2017 so for a limited time only.
The Bottle with a Note item was introduced a week after the Margoria expansion in the EU/NA Region with the 01 February 2017 Patch Notes which included this;
As the notes stated, the item can be obtained while harpoon fishing. It leads onto various versions of the Gardaka’s Treasure Map which starts an associated quest depending on which version you get. The quest is to solve the enigmatic riddle included in the map tooltip text.
However, while exploring potential Kamasylvia fish species along the border of thar region due to beww opened here sometime in March, I found a small pool which contains a Kamasylvia fish table (including the Swiri), to my surprise I fished up one of the bottle items. So I can confirm that it is possible to also fish up the Bottle with a Note item as well as via harpooning now in-game.
The bottle functioned as expected and started the quest. Thinking it may be a bug I continued to fish and a day or so later fished up another which confirms that it is possible now in-game in our region to obtain it via fishing and not just by harpooning.
Checking further, I have now found two other references to these being fished up in Kamasylvia from the Korean forums so it looks very much like this is a deliberate design decision to add it as a rare drop probably in any Kamasylvia water body. The reason we can catch them before the new region is opened is because the fish table overlaps a bit into existing land in Calpheon. This happens occasionally, like for Margoria before launch etc.
The only water body I could find with another fish table other than that already posted on, accessed over the border into Kamasylvia from the Ross Sea, was this small pool found to the SW of Trent. It is in the Cyclops Land area (on the mini-map) and why I refer to it as the Cyclops pool. The location is indicated on the attached image being above the Ksarka Shrine (Calpheon).
The pool can be accessed by moving SW out of Trent from the storage keeper, up the hill and then either side of the shrine entrance steps. Or up those steps past the shrine NPCs and jumping off and climbing the rocks to reach the pool set back a little from actual shrine entrance.
Players should be aware that although fishing gives probably a more realistic option for obtaining a bottle item over harpoon fishing (the drop rate may vary etc.), I have yet to see even on the Korean forums any actual confirmation of one of the actual treasures being located by solving the various riddles.
In time I expect the locations will become common knowledge and having something hard to solve is really cool in itself, currently it will require a lot of communal detective work to try and work out the locations and gain the Golden Chest reward.
I can also confirm that only one quest can be accepted at a time and the bottles and treasure maps are bound items, so until the current quest is completed you wont be able to remove the item from inventory or it will be forfeit as the item tooltips state.
For more detail on the Swiri found here also and fishing table generally please check out the separate thread here.
Good luck with fishing one up (or harpooning) and join the world hunt for these treasure locations.