Blue trash bags & bubble wrap. Walmart carries them, sometimes they're recycling bags, but one brand is actually blue.
You could just cut a couple leg holes in the bottom for your legs, bring it up, put in some arm holes, tie it up around your neck after you've filled it with bubble wrap. If you could find blue bubble wrap great, if not clear would work, especially if the trash bag is a clear, recycling type of bag. If the bag is opaque, you could stuff it with anything, newspaper would be easy.
The other option could be the cardboard/foam route. Just cut out a droplet shape, paint it blue or cut it out of heavier blue paper & attach it to your blue outfit.
Like this kid:
OK went down the google rabbit hole, this is super cute:
This is a variation on the theme:
Here's a whole page of pic links with kids as water droplets:
Great question! That was a fun search that I thought would result in nothing or weird stuff!!