G'day Toni,

Yep, these would be suitable for that purpose.  They're just foam interlocking squares, so no issues with them to getting wet.

G'day Bev,

They'd do the trick nicely, it's a common use for these mats!
It's a good idea to join a few together so you get inside and outside the shower covered, which helps keep your feet clean.

G'day Levi,

These are a popular option for swag users who want something different from a traditional ground sheet.  Its not going to give you guaranteed water protection, so if thats an issue and you dont have a waterproof bucket floor on your swag, you'll need to consider throwing a water proof layer ontop of the foam matting.  It will add abit of comfort and warmth.  Another thing to consider if you need additional warmth is a Coleman Stratus Fleece Liner, not as a liner to sleep in, but if you unzip the foot it can slide over your existing swag mattress like a sleeve (depending on how big your mattress is), it can also increase comfort and give abit of extra insulation.  Hope that helps!