This is a 'clean' image with less variation in the RGB channels than previously

So I think if we replaced the Blue channel with the Red on, we would marginally (1/6th) improve the detail in the boats and also darken the sky.  In addition a small contrast curve in the Green channel would bring out some more detail at the expense of the left foreground and church.

Now I just do not like the idea of doing these corrections blind, i.e. without seeing the b&W image as I make the adjustments.  So I had the idea of creating an additional view (Window>Documents>New Window - in Photoshop) and setting that view to Proof Colors with a profile of Gray Gamma 2.2.

Here is a screen shot of this setup

Note that the b&w image is shown as Proof and so when you de-saturate the image, it will slightly change - which is very irritating after all ones work when watching the corrections!  The solution to this problem is to convert rather than de-saturate (Image>Mode>Convert to Profile... and then select Gray Gamma 2.2).  So here it is...

At last I am now feeling happier about converting to b&w and being able to see what I'm doing.

An alternative, if one does not want to view the coloured version while correcting, is to add a White background layer above the Luminosity layer and then de-saturating will be equivalent to the view shown. Thus...