"là" here emphasizes the word "thật" (really) . So the sentence should be accepted without "là".
Interestingly, là is a copula, meaning it is usually translated as "to be". It doesn't work exactly in the same way than in english.
Những đại dương thật là lớn. - the sentence would be glossed as: [Some] [Ocean] [really] [to be] [large]