Players can find the orange, yellow, green, blue, purple, red & pink message bottles by finishing several quests. Then use the hints from the bottle to dig up the Conch at their respective locations using a Plastic Shovel.
To open the big grill hatch inside the Sand Castle, you must collect the seven different Conch, which in turn requires you to collect the corresponding bottles. So, if you are wondering how to find each one, here is a detailed guide.
Where are the Bottles & Conch Located in Fisch?
Below, you can find all the Bottles and their respective conch locations with GPS coordinates. Just follow them, and you can find each one in turn.
Yellow Message Bottle & Conch Location
The first and easiest bottle you will find in Fisch is the Yellow Message Bottle. It is right behind your spawn point, near the beach coast adjacent to the Bonfire Blake NPC at GPS (-23988,2593, -4445).

Read the message in the bottle for the hint, which reads as "To the island across the main, and check the far edge for what's left behind."

Now, make your way to the exact spot at GPS (-24477,2656,-4732), which is next to a blue building right at the start of the Coconut Town. Equip the Plastic Shovel and start digging to acquire the Yellow Conch.
Orange Message Bottle & Conch Location
For this one, you need a Driftwood, which is a Trash Tier item fishable anywhere on the map using the Magnet Bait. 59Once you have some, head to the Driftwood Bonfire at GPS (-23977,2593,-4441). Place down a Driftwood and then speak with the Bonfire Blake NPC next to get the Orange Message Bottle.

Open the bottle to read the message: "The fire's friends sit close to keep warm, but one of their logs is hiding more than splinters."

Now, travel straight to the Sand Castle Hatch area and dig at the location (-24048,2603,-5113) to obtain the Orange Conch.
Red Message Bottle & Conch Location
The Red Message Bottle is also pretty easy to get: just participate in a Jet Ski racing competition and finish the race within 2 minutes. Go towards the Dock area to find the Jet Ski Referee and start a race with other players. When you finish it, you will receive the Red Message Bottle as a Reward.

Reading it will reveal the clue "Where the ledge dares to float above the water, the final mark is waiting for you."

Now, go towards the north-eastern side of the map to find a lonely lamp post at (-24130,2598,-4868). This is just the opposite of the Shipwright NPC. Dig towards the right side of the lamp to find the Red Conch.
Green Message Bottle & Conch Location
This is a slightly challenging Message Bottle to obtain, as mere participation is not enough, but requires you to win the Sand Castle mini-game twice! So, go to GPS (-23965,2592,-4654) to start the mini-game. Create a Castle exactly similar to the one shown to you to win it. After two victories, you will get the Green Message Bottle.

Open the bottle to get the hint that says "The castle of giants keeps its secret tucked on a wall, not out in the open."

This hint actually points to a location behind the Shipwright NPC at the northern edge of the map (-23994, 2596, -4823). Here, dig using the Plastic Shovel to get the Green Conch.
Purple Message Bottle & Conch Location
The Purple Message Bottle is another easy one to acquire. Go to the Sunshell Merchant inside the Coconut Town (- 24498, 2647,- 4423). Interact with the Merchant to get a list of items she sells. Select the Purple Message Bottle and pay 6K Sunshells to get it.

Equip the bottle from the inventory and open it to receive a message that reads "Treasure rides in coastal crates, and one of them wears its message right on top."

Now, go towards the lonely house next to the canal that leads into inner parts of the Fischfest 2 Island at (-24274, 2595, -4444). Dig at the marked location next to the house to receive the Purple Conch.
Blue Message Bottle & Conch Location

To get the Blue Message Bottle, visit the Tiki Hut at GPS (-24014, 2594, -4480). Here, interact with the Drink Server and purchase a Random Drink from the menu for SunShells. While doing so, there is a 1-in-6 chance of getting the Blue Message Bottle as a reward. Keep purchasing a drink every 15 minutes until you get it. Also, remember, each drink costs you around 25 Sunshells. So, save up a lot of Sunshells to keep getting the drinks.

After opening the Blue Message Bottle, you will get a hint about it that requires you to travel to a lonely piece of land at (-23591,2611,-4350) opposite the Fischfest Island adjacent to the Lighthouse. Dig at this exact spot to get the Blue Conch.
Pink Message Bottle & Conch Location

There is no Pink Message Bottle that will give you the Pink Conch. Instead, you must finish all the quests related to the Captain Conch NPC standing opposite the Tikki Hut. He offers multiple quests that range from participating in mini-games to catching a particular type of fish. Finish each one of them to get the Pink Conch as the final reward.
How to dig up a Conch in Fisch
To dig up a Conch on the Fischfest 2 Island, you require a Plastic Shovel. This is an item that players can find along the coast of the Island or catch from the sea during the summer season. Either method works, and you need it to open the hatch inside the Sand Castle that will take you inside the Heart of the Island.
For more on Fisch, check out All 34 Fish in Fischfest 2 (100% Bestiary) - Fisch or All 10 Crab Locations in Fischfest 2 - Fisch