Blue Swimmer Crab
Admittedly this Blue Swimmer Crab isn't blue and it is not swimming, but we can assure you that they normally are blue and they can swim, we just made do with this photograph.
These crabs spend much of their time buried in the sand (probably because they taste so good). It can be a bit of a shock when you accidentally uncover one as they can get to a decent size. It's even more of a shock when you see one swimming, especially when they're heading straight for you as their pincers look much larger underwater.