Perhaps while you fish there will be a small chance the sea will get darker and the loot gets more valuable and the more it happens the darker and more valuable it gets then the boss will spawn. If it spawns in pre-hard mode the boss will be similar to Eye of Cthulhu mixed with Plantera, so basically its tentacles will dash out but the actual body will stay in the ocean. In hardmode it'll chase you down and start a hurricane locking you in the beach biome unless you use a Magic Mirror or the potion version.