The Seabound Shrine Interior isn't terribly different from other dungeons at first. Begin progressing through the maze, defeat terrors that appear, and after following the linear path you'll soon reach a Mila statue.
Beyond the Mila statue there are Sacred Springs – once they've dried up, look carefully. There's a doorway blocked by breakable stone between the two Springs. Break through and progress.
As you continue through the maze, what you're ultimately looking for are more Sacred Springs. You'll eventually reach a room with more breakable doors, and these Springs are contained within. One offers waters of Resistance, the other offers waters of Experience. The former works as the other Springs do (adding a single stat point per drink) while the latter affords 100 experience points per drink, to be used on a single unit each time. As such, it's a great chance to level up lagging units.
Progressing further reveals a chest with a Blessed Sword inside. Once collected, return to the Mila statue or leave the Shrine.