I am a Tofino resident, guide, tour company operator, and surfer and have spent considerable time with my feet in the water on our local beaches. I have also been nibbled at by these little creatures. After checking in a few of my marine encycopedias, it's likely that the creature we're speaking of is "Cirolana Harfordi" aka: scavenging isopod, harford's isopod and other names.
These little isopods keep the beach clean! They eat, or attempt to eat, anything they come across in the intertidal zone. My tour company used to do salmon barbeques on the beach, and after we were done, we would put the salmon bones, tails etc into the water, and these isopods would gather in large numbers to eat our leftovers. They would of course, also try to nibble on my feet if I stood in the water long enough, but they never broke the skin. I imagine if you stood there long enough they might, but I didn't do the experiment. Their mouthparts are quite small of course, so a pair of aquasocks or neoprene booties would certainly stop you from feeling them, and they likely wouldn't try to bite the neoprene anyways.
In the instances that they've nibbled on my feet, I've never had the type of reaction that is typical as a result of a mosquito or blackfly bite. The "bite" appears to leave no mark, and results in no swelling, but it is mildly painful, perhaps akin to gently pressing on your skin with a pin, without breaking the skin.
No deet required! I sort of doubt deet would even work for very long after being submersed in sea water. It would probably wash off fairly quickly. It should also be noted that these isopods are not present at all beaches, all the time. They seem to be around only some of the time, and then gone. Your chances of being at the spot where they are are relatively low I would say.