We did an August inside passage cruise many years ago on Infinity and had an aft cabin. We were excited to have the big balcony, but the rainy weather kept us indoors for much of the cruise. One amazing thing was when we went into Glacier Bay. Our view was unbelievable and we were happy to have it. We had a storm blow through near Skagway, and nearly got bounced out of our bed as the side thrusters turned the ship when they realized it would be too dangerous to attempt docking in the high seas. IMO there’s more motion in the aft cabins on M-class ships than E-class, and there were times on Infinity we could feel the vibration of the engines while underway, on deck 8.
Also note that even when the sun is out, the veranda can be fairly cold from the wind of the ship while underway. This is where an aft cabin can be an advantage.
August and September tend to be very rainy in Alaska, so you’ll want to plan for that strong possibility.