Dungeon Crate

Every one of these carries a Golden Lock Box, and that box rolls against the same pool as the locked chests built into the Dungeon. A world only generates so many of those chests. Once you have cracked all of them and the Dungeon weapon you wanted never showed up, fishing is the only supply left in that world. Everything else in the crate is packaging around that one line.

Skeletron has to be dead first. Cast before that and you get ordinary catches no matter how patient you are. You also need to be underground with natural Dungeon wall behind you, the backdrop that generated with the building rather than any wall you put up yourself. A pool dug into a room that still has its original walls produces crates. A pool a few tiles outside the brickwork produces nothing special.

Fishing in an open corridor means fighting between casts, and the residents hit hard for a character who has only just put Skeletron down. Pick an isolated branch instead and pack it solid with dirt, hay, or whatever you have in bulk, so there is no open air for anything to spawn into. Carve a small niche at the outer edge that leaves the original wall exposed at your back, and put the water just beyond it. The game still reads you as being inside the Dungeon, so the loot table holds, but nothing has room to appear beside you.

Colored bricks and paintings come up constantly and only matter if you are decorating, though a few of the paintings are awkward to find any other way. Books pile up fast and are worth keeping for shelving. Ores, bars, and utility potions are the same filler every biome crate carries. Do not bin the Journeyman Bait and Master Bait, since you are going to be casting here for a long stretch and that bait is what keeps the rod producing.

The contents are locked to pre-Hardmode regardless of when you pop the lid, so a chest full of unopened crates is worth the same before and after the Wall of Flesh. There is nothing to gain by saving them. The Stockade Crate is the separate catch that covers the Hardmode Dungeon, not an upgraded version of this one.