How do you get the algae filled ocean...cause..I bought agriculture right away...So..they stopped fishign and onyl farm and get cocunuts now..

Overfishing.

And how do you overfish?

Keep sending villagers to catch fish.

But they won't go because the water is dirty! I have level 2 in agriculture. Is that the issue?

You need more agriculture and some master farmers and reseachers

Eta, I think you misunderstood the original poster's question. They were asking how to make the algae appear in the ocean, not how to get rid of it. The algae appears because of overfishing, but once it appears you must solve that puzzle and clean up the water, which, as tkdgrl223 has explained, requires Level 3 of farming technology and some highly skilled villagers.

Thanks guys! My villagers just died (all of them).
I am starting again!!
I'll do it, I know!
Thanks to you all!!

Oh, bummer! I'm sure it will go better next time.