You guys probably already knew but I just found this out. The recent BBC nature documentary Our Planet is available on Youtube for free! Some episodes of interest are:
Coastal Seas: https://www.youtube.com/watch?v=r9PeYPHdpNo
High Seas: https://www.youtube.com/watch?v=9FqwhW0B3tY
but the rest are just as good.
Edit: I lied, this series was released last year. The newer one is called "A perfect planet". You'd have to watch it on BBC if you have access (VPN) or take to the high seas yourself (thepiratebay).