No, they aren't the quality of SS that is slatwater "safe". They make plastic clamps of all different sizes that are safe in salt water. Home depot used to carry them, didn;t see them there last time I was looking. Locally, check lowes or maybe your home depot otherwise just about any online place that sells stuff for aquariums or plumbing should have the plastic clamps. www.aquaticecosystems.com definitely does.
Try buying your stainless steel clamps from a boating center. They are the same price, and the screw is actually stainless as well. The ones you buy from HD or the autoparts stores have a mild steel screw that rusts QUICKLY in our environments. I have several that I got from a boating center that are a couple of years old, and doing just fine. They sit on top of my sump and are exposed completely. They are designed to last years in a boat bilge in saltwater.