the document is too big in size, that GIMP could not blur it, how should I do ?

[hobbit604]

Good question - I think this is due to a change in Gimp as it was upgraded. In the older version, whenever you made a new layer it gave you the options of 'white, black or transparent'. In the latest versions it is called 'foreground, background or transparent'. Assuming black is your default foreground colour and white is your default background colour, you can see that it's pretty much the same.

In your case, make sure that black is selected as either a foreground or background BEFORE you make this new layer. Then, make a new layer (it does not matter whether this is at the top or at the bottom of your layers because it is just temporary) and in the options select either foreground or background colour - whichever is currently black. The new layer will appear filled with black. And then you just fill your inverted-sea selection with white, so that all the land goes white and the sea stays black.

sorry to ask you again, but I'm so bad at the computer skill, I couldn't find where is the option to select colors for the Foreground and Background layer