I haven't seen a serious reply to some of the practical questions you ask, so here's my put.
As to how shallow you can run, I don't think there's that much difference between sleds. I have a 19 ft North River vee bottom that I can take through 4 inches of water if needed. You may be able to get a flat bottom sled through 2 or 3 inches, if needed. Problem is, if the water is that shallow, there is almost certainly at least one rock that sticks up, and you're going to bang it. So I think you're better off with a boat that is designed for some abuse, ie, a real sled, than a Smoker craft with a pump on it. The sleds have more metal on the bottom.
Now, if you're reasonably careful, any more or less flat bottomed boat with a jet unit will work to get you up and down the river, and a Smoker Craft Alaskan with a 20 hp pump might be just right for you. With the bigger boats, you get a nicer platform to fish from, and maybe amenities such as nicer seating, etc. A lot of Alaskan guide outfits do real well with 16 foot Lowes with about a 25 hp jet unit. The Lowes aren't heaviweight boats, but they do they job.
On the Sky up to Sultan, I have never seen the depth finder go below about 2 feet when I'm running, but I know how to read water and see the slot, and I just don't run the powerboat at low water levels, ie, less than about 2300 cfs at Goldbar. That's actually due as much to the conditions at the ramps as anything else, however.
You can run these rivers just fine in the summer with any of these boats. I think the limit is not the river, it's your ability to read water, your familiarity with the water, and the other people on the water. In the summer, the river is smaller, and so there is less room for co-existance with non-powered craft and bankies. I don't run the big boat in summer water on the smaller rivers, because I think it is getting pretty close to being rude to my fellow angler. I think it's a borderline question as to whether the Sky qualifies as a smaller river.
You'll get some folks that will tell you that too big a boat throws too much of a wake. I don't think that size is as much a factor as design, weight, and how the operator uses it. My 19 ft throws less wake than my old 17 ft, despite being 700 lbs heavier and having 100 more HP. I think it's because it gets up on step easier, and keeps on top, rather than plowing through, the water.
If you get a boat, make sure it has enough power for the boat, you, and the number of people you plan on fishing with it. Having a pump doesn't mean squat if you can't get a boat on plane. I had a 17 ft Almar with an 85, and with more than two people, the boat simply wasn't safe in shallow water, as it took about a football field to get up on step. If you find someone selling a boat for a price that seems too good to be real, it may be because the motor is too small for the boat. Insist on taking it out for a spin, and bring buddies, large ones.
So that's my advice. As to whether to get a drift boat or a sled, I actually prefer fishing from a drift boat, as I love to row, but a drift boat requires a shuttle. I like to fish with my kids, they don't have drivers licenses, so the sled works for me. I fish in mid week a lot, so finding partners who can help with the shuttle can be hard. Note, volunteers accepted. It is also nice to be able to put in, motor up to a spot, fish for a bit, and then head back, without having to do the whole drift. I have a couple of spots where I fish where I never fire the big motor up except to get on and off the trailer. I just put in, motor up to a point, anchor and plunk. Being able to sit there, drink coffee under the top and wait for mister steelhead to come, now that's luxury.
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