More closeups! "Amphipod" is kind of a catch-all term for insect-like water residents – it applies to the isopod here, as well as to this guy, though you can see the resemblance isn't terribly close. But far more often, these little beggars are simply called "scuds".
Scuds can be found on virtually everything in the Indian River Lagoon – pick up a rock, shell, some seaweed, even a horseshoe crab, and you may find several of them crawling across your hands. While a creepy feeling, it doesn't pose the least hazard, since they're herbivorous and non-parasitic.
But since they also form a food source to lots of different animals in the water, I tried to keep a decent supply of them in my tank, which was easy to do based on their habits outlined above. This one shown here is average, about 4mm long. and I like the detail that can be seen in the eyes.