He was flailing his arms

[99bottles]

Context: Drowning

Author: Me

He was flailing his arms as his body was sinking, with him being unable to do anything to stop it.

Is a comma needed between sinking and with?

 

[Tarheel]

Yes.

Who or what was doing the sinking?

 

[99bottles]

Yes.

Who or what was doing the sinking?

His body was sinking into the water.

 

[Tarheel]

I wouldn't use "His body" there (if at all). It makes it seem like he's disconnected from his body. Instead, he was sinking.

Sinking into water doesn't take long. I've done it myself--usually on purpose.

Gravity makes a difference. If you jump off a bridge into a river you will plunge under the surface.