You're dreaming if you think $200 bucks is all that this trip will run you.
It'll take you quite a while to run from Sewell's out to the mouth of the Frasier, quite a bit longer than you think, there goes between 1/4-1/2 of your half day trip, and quite a bit of gas. So a full day trip is probably your best option, which brings your total up to $500. (Not to mention I don't think you're allowed to take a Sewell's boat out that far)
Assuming you already have all the necessary gear for Sockeye (Rods, Hoochies, leader, etc). Those boats still don't have downriggers, so you would have to run sliding weights, or mooching weights to get down there (or a diver), which if you've never done it before, is very annoying. Your line will be quite a distance behind your boat, so you have to be extremely careful to not get tangled up with one of the millions of other boats out there. This is not ideal, especially if you're not an experienced boater. I'm not even sure if those boats have rod holders, and holding on to a rod while trolling in the chuck is not the most fun way to experience the sockeye fishery.
Why not get a half day guided trip, which would run you about $600 plus tip, you're guaranteed to limit out on Sockeye, you might even get a chance to target some Springs, and you don't have to worry about ANY of this crap. So you get a stress-free trip where you can knock back a couple of brews, and you're guaranteed some awesome fishing that your friend will be telling all his friends at home about.
I'm not affiliated with any guides, just trying to help. There is a VERY high chance that if you rent a boat that is ill-equipped for fishing, that you'll have a day full of tangled lines, frustration, and may come off looking like an idiot for your out of town visitors.