I am having hard time to find nature symbols for polygons. Any tips to find more symbol (any plugins?)
Actually that set of styles mentioned by @underdark was improved couple of months ago and new set do contains swamp styles. I was going to translate captions of symbols to English (original are in Russian) and post new link at my blog, but just can't find time for it(((
Nevertheless, here you are the link for the upgraded style set with swamps fills. I bet captions in Russian won't prevent you from utilisation of the needed styles.
Some natural polygon fills are available at http://ssrebelious.blogspot.com/2011/10/qgis-symbology-set.html. I didn't see a swamps style though.
I found a couple of built-in options in QGIS 3.4.2 Madeira:
Option 1) A built-in fill pattern. Go to layer Properties, Symbology tab, Fill (note: you must click "Fill," not "Simple Fill" below it); then choose "All Symbols" and go to the bottom, where there is one called "Topo swamp." It's in the Topology symbol group.
Option 2) An SVG fill. This is more flexible, and it's easy. Go to layer Properties, Symbology tab, Fill, then highlight the second line, "Simple Fill", and below that in "Symbol Layer Type" box select "SVG fill" from the dropdown arrow on the righthand side of the box. The menus will give you additional options when you change to "SVG fill". Under the symbol group called simply "symbol" there are a couple of swamp items.
A couple of symbol choices are shown on the end of the first line/beginning of the second.
Note that both options allow you to change the sizing and packing density of the symbols (also the usual other qualities like line color/type, background, etc.), which is quite nice. I couldn't get the Russian SVG set to load.