Blown sand on the shoreline, sand inland for that matter, or even strong wind with dust would be bad for the coatings.
Water - they keep water marks off your front element too - saltwater does nasty things to your front element, it takes the coatings OFF, so don't let salt spray get onto yours - I shoot powerboats, boat to boat, so getting saltwater all over my gear is pretty much a day to day occurance. A poly bag keeps the worst off the outside, but you need the end of your lens stuck out in the elemnts to get any kind of a shot! Shooting through a polybag degrades the IQ somewhat.
Hoya Pro 1 multi coated ones are fine - the B&W are good quality, but about 3x the weight. Cumbersome would be how I would put it. Very good quality, brass mounts, excellent glass and coatings. The Hoya ones are much thinner - more delicate even - and much lighter. I don't have a lens without one on, except the fisheye!