bad luck of the sea

There can be a status effect called bad luck of the sea.

It lasts from 10-20 minutes and if you try to fish in any body of water with this status effect you have a chance to receive: 

a lit block of TNT 

a splash or lingering potion of poison, harming, slowness, or weakness

a drowned zombie

a guardian

(less likely) an elder guardian

an arrow coming toward you and damaging you

(less likely) an arrow with a status effect of poison, harming, slowness or weakness

These arrows also have a small chance of having healing, swiftness, or strength

any armor fished up will be one hit away from breaking and will not include any enchantments

Milk cures the status effect.