I haven't done much from a boat, but the salmon fishing I have done in the Sound tells me you don't have to get anywhere near 50 feet down to catch coho in places where that's doable. Indeed, I don't own any gear that would fish that deep with a fly line, and if I did, I suspect it would fish and handle like shit, and I would rather use conventional gear in that situation anyway. Fly fish where practical and deploy the gear everywhere else would be my mantra in the Sound. Even with sinking lines, fish caught on fly gear are almost always in the top 10 feet of the water column, and I have caught most of mine in the top 3 feet or less.
Certainly, others have had other experiences, but I doubt seriously anyone's hooking salmon 50 feet down with a fly line on the regular (would be interested to hear if anyone is!), so you probably don't need to set your sights that deep....