The Tidal Bell is a Key Item exclusive to Alpha Sapphire. You get it from Captain Stern when you give him the Scanner in the Slateport City Harbor.
In Alpha Sapphire, the Tidal Bell is used to encounter Lugia in Sea Mauville. While its counterpart, the Clear Bell, is used to encounter Ho-Oh in Omega Ruby.
Obtaining the Tidal Bell
Firstly, you will need to get the Scanner from Sea Mauville. This is located in the fully submerged control room, which is only accessible by using Surf and Dive.
Once you have the Scanner, you can return to Slateport City. Then just speak to Captain Stern to give him the Scanner, and you’ll get the Tidal Bell in exchange.
Here’s where you can find Captain Stern:
Step 1: Start at the Pokémon Center in Slateport City then head northeast.
Step 2: In the northeast corner, you’ll find the Slateport City Harbor building. Go inside.
Step 3: Speak to Captain Stern (green hat) and you’ll get the Tidal Bell in exchange for the Scanner.
How To Use the Tidal Bell
The Tidal Bell is primarily used to encounter the legendary Pokémon Lugia, exclusive to Alpha Sapphire.
Once you obtain the Tidal Bell, simply return to Sea Mauville and go to the same underwater control room where you found the Scanner.
Note: You can only encounter Lugia after defeating Primal Kyogre.
Once you’re in the control room, head further down the stairs past where you found the Scanner.
You should find a portal there.
Tip: Save your game before entering the portal. If Lugia faints, you will have to battle the Elite Four and the Champion again for it to respawn.
Once you enter the portal, you will encounter a level 50 Lugia.
Tip: In the Omega Ruby version, you will instead get the Clear Bell from Captain Stern. Returning to this same spot in Sea Mauville lets you encounter the legendary Pokémon Ho-Oh.