How come ocean water is not contaminated with blood and poop of aquatic creatures? ?

There is poop and blood in the ocean. But that’s like putting a drop of blood in a swimming pool. It’s such a small amount compared to the water that it makes no difference. The salt kind of kills all the bacteria anyway. 

The water close to shore where all the people live and play is contaminated with huge amounts of sewage, medical waste, dead bodies of birds/fish/mammals and industrial waste. 

We tried to solve that problem by dumping plastic waste into oceans to absorb all that blood and poop. It must be working because we don't see blood and poop. Just lots and lots of plastic!

It's not just ocean water, it's also in the lakes you get your drinking water from.

Feeling thirsty?

because there are organisms digesting those

It is not contaminated because that implies they would pollute or posion the oceans, which they do not. The faeces and blood of marine life in the oceans is normal.

There is a lot of ocean when compared to the quantity of faeces and blood so it is in effect diluted.

Plus what is there does not remain there because it is decomposed.

It isn't a contaminant when it is the normal state of things, just so you know.  The normal state of things is that there is a minor background level of organic debris of many different origins including excrement, but such debris is impermanent, so decays away relatively quickly, and concentrations never become high.

Besides, the ocean is huge and life is a tiny, tiny proportion of it, so even tinier amounts of dead stuff or excretia that are added are really tiny in proportion relative to the life form from which it comes.  It definitely would become a problem if the stuff never got destroyed, but it does, so there is no "contamination" except locally for short time periods, if at all.

Salt, on the other hand, does not decay, and the seas are salty because of addition of small amounts for extremely long times.  See the difference?

It is, but the sheer volume of ocean water is MASSIVE compared to the blood of a fish.