It would rust, unless painted/protected. That doesn't seem much of a problem. How it was anchored could be an issue. One problem with any launch is that it can stop sediment movement so it should either be on legs/pilings or laid directly on beach.

Depending on the size of boat and length of launch needed it might work best to have a "portable" launch. Out in spring, back in for winter.

Ideally, and many of my friends say I live in a fantasy world anyway, you should have a local regulator (WDFW Habitat, County, City) to brainstorm a solution for that location. How can I get my boat in the water in the best manner for me, the beach, and the environment. I my experience, this yielded some of the best solutions to what had been very intractable problems.