The double gerund phrase (sinking…,keeping…,) in the middle as a subject of your verb (does) gets confusing. It’s hard to tell from this long, complicated sentence exactly WHAT does not allow the diver to breathe or what water pressure has to do with it.

Break it up into 2-3 sentences that explain more clearly:

1. the situation (hollow cylinder with its top above the surface),

2. what water pressure does to it, and

3. why the diver can’t breathe through the cylinder.

Not necessarily in that order. Also, check your spelling.

Due to the pressure of water, sinking a hollow cylinder down to ocean floor, but keeping the top above the water surface, does not allow a diver underneath the water to breathe air throught the cylinder?

Though most of that doesn’t make any sense to me, I changed the start of the sentence for you.

Sinking a hollow cylinder down to the ocean floor while keeping the top above water will still not allow a diver to breath due to the immense pressure of water on the cylinder.

How about this as a replacement:

Due to the pressure of water, sinking a hollow cylinder down to the the ocean floor, but keeping the top above the water surface, does not allow a diver underneath the water to breathe air through the cylinder.

Does not allow a diver underneath the water to breathe air through the cylinder, because of the pressure of water, sinking a hollow cylinder down to ocean floor, but keeping the top above the water surface.

That doesn’t make any sense, so you would have to re-word your sentence. Because you will re-word it, the sentence will make sense.

Just had a thought….who makes up these word rules?

The sentence you have there is a bit confusing to read through, consider revising it.

Here’s an example of what I’m talking about:

Becuase someone asked a question, she got an answer.

She got an answer becuase someone asked a question.

You could switch it no problem, see what I’m saying.