Thanks for the nod Thomas712,
With regards to stainless, you can submerge a hose clamp, and it will likely do its job for many years.
I bought my 92 reef used, and the guy who owned it before me used stainless hose clamps on everything in the sump. They were rusted, but still functional and intact. He had the tank set up for years, and had great success in reef keeping.
My judgement on placing stainless under water in your tank is this: It probably won't dissociate any iron, chromium or nickel into your tank, but why risk it. If the connection is underwater, you can usally just ase a tight fitting and it will hold together. It may leak a very tiny amount, but so what?...its underwater!
If its above the water line, it will likely get corroded in the salt rich air. It will remain functional for YEARS though.