"Due to the vast amount of knowledge"

It's narrated in Hiliyat Al-Awliya:

قال مجاهد رحمه الله : كان ابن عباس رضي الله تعالى عنه يسمى البحر من كثرة علمه

ENGLISH:

"Mujaahid said: Ibn Abbas, may Allah be pleased with him, was known as "Al-Bahr" (the ocean), due to the vast amounts of knowledge he possessed".

NEDERLANDS:

"Mujahaid zei: "Ibn Abbas, moge Allah tevreden over hem zijn, werd "Al-Bahr" (de oceaan) genoemd, wegens de hoeveelheid aan kennis die hij bezat".

["Hilyat Al-Awliya", 1/316].