As I've posted a lot of times, what I like to do with potatoes on the grill is cut in half (if small) or cube them, toss them and some onions and peppers if handy with some oil and S&P, or rosemary or garlic or whatever you like, seal in a foil pouch, and toss it on the lower rack in the WSM or one side of the Weber. As long as it's not over direct heat, should cook to tenderness in an hour with absolutely no attention from you whatsoever. No, maybe you don't get the crispy-grilled-potato thing, but they sure come out nicely and let you focus on not screwing up the meat.
You know what else would go great with the fish or the pork, especially, right now when they're at their best of the season? Melt some butter and brown sugar together, slice peaches as best you can into whole slices, brush with the butter-sugar, and grill lightly on a very clean part of the grill that won't transmit a meat flavor. An old Southern thing, but rarely done up here, and as good as baked apples with pork or a fruity salsa on fish.