I was intrigued to see this effect and so I tried it on my map with some interesting results.
The one I thought was most interesting was when I selected channel A and Linear Light blend mode. It left my landmass pretty much alone but gave my oceans a nice blue colour, like a light ink wash.

I think it looks quite cool.

Click image for larger version. 

Name:	Vaniya16 - alternate.jpg 
Views:	296 
Size:	2.48 MB 
ID:	13634

Incidentally, I'm not sure why (I'm sure someone could tell me) but when I view an image in PS the colour range is significantly different to when viewed in Windows picture viewer or in Firefox. For instance, the image I have posted seems to lose some green in the transition, the new blue colour for instance seems to lack a greenish tinge that shows in the original in PS which is a shame. I imagine it is something to do with the "Color Settings" in PS but I have no idea what they should be set to. :S