This is pretty much it - also, don't use any of those water/steam brushes (that you fill the handle with water). Water on the cast iron grates results in a need to season the cast iron. Until you do, everything will stick.
Years ago I worked as a broiler cook in a seafood restaurant and would stand and grill fish for hours at a time - probably a hundred+ pieces a night. Nothing like standing over a scorching hot grill for 8 hours with a sunburn and a hangover.... ah, to be young again
I've had the best luck with canned spray to lube the grill and the fish but be careful spraying on the grill as it will flame up.
Tender fish like Cod or Sea Bass is always a tough thing on a grill and I'll just put them foil, etc.. Salmon and Halibut are a bit more firm and hold up okay. All the tropical fish (Marlin, Tuna, Swordfish, etc.) has a nice firm texture and is generally the easiest to grill without sticking/damage.
Salmon and Halibut I tend to just go with dry rubs/spices - often Cajun of some type. But I mostly eat tropical fish with some kind of marinade. I'll generally use some combination of the following - minced garlic and/or onion, minced lemongrass, Asian fish sauce, oyster sauce, soy sauce, sesame oil, small amount of fruit juice.
One of the quick/easy ones I do a lot is chopped onion, oyster sauce, just a couple drops of sesame oil and a small amount of POG (passion fruit, orange, guava juice).
Unfortunately, my family doesn't care for fish or I'd eat it every night. I still get plenty though. I live part time on Maui and the Costco here has a great supply of local fish.