How do I get into the warehouse at the docks?

You'll have to satisfy Watson by looking through the window.

But the window is too dirty to do you any good right now.

Did you notice the pail above the door?  If you could reach it, you might be
able to clean the window.

You need to climb something.

Push the crate and stand on it.

Don't forget the rag.

Push the crate back to where it belongs and you'll find it.

There's a huge source of water nearby to get that rag wet.

The Thames, of course.