Even with a the heaviest versatip, I was never able to get a fly more than a foot deep. The versatip would sag I a 'u' shape, but I could still see the fly at a foot deep. Do I need to buy a sinking line?
Nope, with the type 6 or 8 versi tip you should be able to dredge the bottom in 4-6 feet of water fairly quickly unless the flow is fairly high. If you can still see your fly is less than 1 foot deep with the heaviest tip, you may have mixed them up (i.e. put on a type 3 as opposed to the 6 or
or you did not buy the heavy one and/or where you are fishing the flow is fairly strong. If the versi tip is sagging in a U shape, it probably means your fly was overly buoyant and large so that it held up the end of the sink tip. Also make sure the fly is getting thoroughly wet and not a dry fly (sorry if this is obvious).
My suggestions can all be used in combination to maximize the sink rate to get your fly down; 1) find slower flowing water, 2) use a weighted fly, either lead wrapped, with a bead or dumbbell eyes, 3) use a spare fly which will be less buoyant and sinks faster, especially if it is has a bead, like a sparse rolled muddler, like this:
and 4) cast upstream to let the line and fly have greater time to sink .
If you have a versi tip system then you will also have a clear (slime line) tip which has a slow sink rate in the region of 1 inch/second. With this line you can get away with very short leaders in the 2-3 foot region, but it does not sink very fast, so only works well on frog water, back eddies etc or in combination with the other suggestions above. I do not typically use this line if water depth is more than 4-5 feet in flowing water unless the flow is relatively slow.
I doubt the mainline is spooking the fish, its more likely the delivery of the line onto the water surface. Coho may be easily spooked by the fly line in still water, but remember when looking up they only see a silhouette against a bright sky so the colour is probably unimportant.
The completely clear lines are often called slime lines and are slow to intermediate sinking lines. They are not used to prevent spooking fish. I have caught pinks and chum on brightly coloured fly lines with less than 3 feet of 15 lbs leader. Trust me they are not worried or spooked by your fly line. Trout in very clear and shallow water may be a different matter, but the colour or clarity of your line is not that important.
If you want to fish deeper in faster water, by all means buy a full sinking line, which you can also use in lakes to great effect. However, you will need another reel or spool and please note that you will initially find it more difficult to cast as you will need to retrieve all the line to drag it out of the water, rather than flick the main line off the surface, and spend more time false casting it back out.
Hope some of that made sense.