One thing I've noticed about spartina, cattails, and other more "stalk-type" grasses is they break down rather quick. It's unfortunate salt hay doesn't grow near you because its super tough stuff.
So Raffia is VERY tough. My three complaints about Raffia:
1) It's expensive
2) It does not stand up straight on it's own. Its very flimsy and just sort of slumps. Salt hay is nice because it stands up and gives a more natural look.
3) Raffia is super messy. I use Raffia on my cockpit cover. It get's caught in my call lanyard, gun, and zippers on my coat and hand warmer.
Even though it's expensive, it's the cheapest of the commercial grasses out there. You can do two whole boats for about $120 (if I recall correctly) and it does last in the weather.