The Erickson Re-Tractable, Ratcheting, Bolt-On Tie-Down Strap, # EM34417, would be a good choice for marine applications. The metal parts are steel but have a zinc-dichromate coating for corrosion resistance. The handle is made of sturdy plastic that will not rust. You should not have problems with it seizing in a marine environment. To pro-long the life of it, you can clean off any salt deposits after use.
Tie-down straps should always be chosen according to their Safe Working Load Limit... view full answer...