does anyone know why there is a message in a bottle (object) on the gray island?

Question

This item is part of the Quest Roshamuul (Lower Roshamuul), but there is one to the left of the entrance to the gray island, would anyone know to tell me if it has any use?

Answer 1

This Message in a Bottle is related to the Falcon Bastion. When you fish it, you get this message:

You find a letter inside the bottle. It says: 'I'm locked in a tower atop a fortress to the south of a foreign continent, help!

If you investigate the Falcon Bastion, you'll find the dead body of a girl in the locked tower to the south. The girl there was visiting the Castle when hell broke lose. She tells that she was locked in there "for her safety" while things were sorted out, but it turns out she's never freed and dies. She tries to ask for help by sending messages in bottles. One of the bottles can be seen floating just south of the island, the other floats to Gray Beach. This can be learned from a diary she leaves there:

Day 1

It was a great feast! We were the first guests of the honourable Falcon Order to arrive. The first time, they invite any delegation to their new outpost. Strangers in a strange new land, it is all so very exciting! We arrived via some form of teleportation, the honourable knights prepared to allow safe passage. The fortress itself is very impressive, more of a bastion really. The great hall filled quickly with countless high-ranking dignitaries from all parts of Tibia. In my function as ambassador and consultant I would never have thought to be invited personally. We stay here for at least another week. Looking forward to meeting the late arrivals before tomorrow's ceremony! When I left our Venore estate, I was sure I'd meet a lot of people - but this is almost overwhelming, really.

Day 2

There was some kind of ruckus during ceremony today: someone spilled wine, suddenly everyone started murmuring and then we heard a scream. We immediately got up from the table and I was escorted away. I'm in some kind of tower right now. It looks like they locked me up - for safety they said. Well, if there's anyone I can trust than it's the dignitaries of the Falcon Order. I can't hear a lot of noises out here but when we hastily walked up to this room I heard screams and the sounds of skirmishing. I hope this will be settled soon enough - what is going on?

Day 3

At least I have food and drink. I won't get far with this wine and the morsels they seemed to have stored in here but surely someone will come pick me up soon. The loud noises and the skirmishes have stopped several hours ago but nothing else happened. Did they forget about me up here?

Day 4

Not a single guard. Not a single dignitary, not a soul. They just left me here. When will they ever return? How many days will it take? I doubt the food I found in this chamber will last more than a few weeks. There's only a single window and it's quite small. If I could only find a rope to get out of here. "Girl, always bring a rope when you leave the house.", my father said. Every single time I wandered off or strolled outside the estate. See? I listened! But never took it to heart it seems.

Day 5

Found a nest outside the window, had some eggs in it. I ate one and saved the others for later. How can I get out of here?

(many of the following entries are dealing with killing time and waiting for rescue, some pages have been torn out)

Day 8

Someone walked by outside, no voices. I could only hear footsteps and it was still too early in the morning to see properly. The dense mist you get here in the early hours didn't help as well. Still didn't find a way to fit through the window. It can be opened easily but it's far too narrow for me to fit through. Well give or take another 10 days and I just might...

Day 9

I threw a bottle with a message out of the window today. Had to sacrifice some of this diary for some quality paper. I still have some wine bottles around here but this was the only bottle I could put a cork in to get it tight enough for a journey over sea. But, alas, somehow it got caught in sea weed or something. I can still see the damn thing right now... I firmly believe my fate is sealed, one way or another.

Day 10 (?) 

Well, I emptied the last bottle of the... red ones. I'm not sure what it was, but I sure can't remember what day it is. At least I got another flask with a cork. Maybe this message will reach someone, it's probably my last chance. No food left for.. how long now?

Day 11

Don't know if I can still see straight, I can't quite make out the second bottle down there so it may have worked... I'm feeling really weak now. For how long...

Day (illegible)

Definitely can't see or write properly anymore. Everything feels distant and I can't seem to hear anything from the outside anymore. There's a body here, lying on the ground. I'm not sure if it was there all the time. I couldn't get it... out of the window... I...

To finally answer the question: its use is flavor on the lore of the Falcon Bastion. The girl's diary sheds some light on what happened there and the bottle is there to make players curious, like you were.