Bloobers are octopus-like enemies which are only found in the underwater sections of Catfish's Maw. Like several other enemies in this game, the Bloober is based off of a similar enemy in the Super Mario Bros games known as "Bloopers". Although they are very weak and can be defeated with one hit, they may be difficult to avoid if they ramming into Link.
Unless they are coming at Link, it may be best to just avoid them. Keep in mind that Bloobers are the fastest underwater enemy in the game. It is best to be below a Bloober rather than above them, because they move upwards faster than they do downwards. When Link is near a bloober, he must swing his sword repeatedly to defeat it.