You can use filters in Affinity Photo to change / manipulate a selection, so the first thing to do is to create one by using the tools such as the rectangular marquee tool or perhaps the elliptical or perhaps use the freehand tool or use the select menu. Go to the select menu and go to the last command in the menu and edit command.
You will see everything outside goes red. You can now also go to the filter menu and apply the blur menu and Gaussian blur or go to the distort menu and deform, twirl, pinch, perspective, equations as well as other effects in the application. They can all modify the pixels thus changing the end result. You can apply them multiple times, you can also use the repeat feature and blends as well as use fades and much more. You can also combine it with other features of the application such as the brush tools and gradients etc to create even more amazing results. Sadly, as the shape tools have no pixel mode you cannot use them.
To go back to normal, just go to the layers panel and click the layer and the red will vanish and you will be left with your marching ants now distorted or modified by the effects and brushes and gradients etc applied to it. You can always go back and re-edit it again and again via the edit command.
You will probably find the area defined by the marching ants is the opposite of the area you want, You can now go to the menu again and invert it and then apply effects such as blur etc. You can also use other tools such as the feather etc to modify things even more.