LR - I've actually done it a few ways. Currently, though, I just bought the new arm from a Sea Ray dealer (the arm, itself, is adjustable for length) where I work part time as the Sea Ray's have been using the J-hook since the mid-2000's and my Sundancer is a '99. IIRC, it was about $30/$35. It's possible that the newer Grady's have this, as well? But this isn't about it being a Sea Ray or a Grady - neither company makes their own wiper motor or wiper arm. With the Sea Ray, it was made by AM Equipment. All you really need to know is how the arm mounts to the motor shaft. It's probably a 1/2" shaft, but it might mount with a screw/nut or maybe just a snap-cover.
Aside from buying one, I've also removed the U-channel part of the arm (the attachment point for the blade) and bolted on a new j-hook tip from AM Equipment. I've even gotten a j-hook arm from a junkyard and made it work. Another way is to replace the slide-out part of the arm with a J-hook slide out. I found that older Mercedes use the same setup, but already have the J-hook.
A friend of mine didn't want to put the effort into doing this himself, so he just ordered a j-hook arm from the internet. He actually went through 4 different sets from 2 different companies (kept breaking) until he finally did what I did.