Not so good.
I'd go with a quality exterior grade plywood, cut to size, laminate pieces together with resin to the right thickness. Much stronger than and will rot much more slowly than dimensional lumber.
Coat it with fiberglass, including resin. Marine paint afterward is optional.
Erik
What Erik said...
It's not so much about the wood being chemically treated for rot. Plywood is going to be stronger than dimensional lumber for stringers..