The answer is, as long as the telescope is at the same height as your eyes, the horizon won't change, and what is visible won't change. It will just appear larger.
So, if your eyes are 6 feet above the surface, and the telescope has a mount that is 6 feet tall, the horizon on the ocean will be exactly the same, almost exactly 3 miles away (2.999547 miles, using the calculator mtmman linked to). You will see the same thing with both, it will just be magnified with the telescope.
So, for #1, the sun will set at the exact same time, bare eyes or telescope (and don't look at the sun with a telescope without filters).
And #2, how far out? With your eyes and the telescope both at 6 feet, 3 miles.
For there to actually be a change to these two things, elevate yourself. Or shrink yourself. Stand on a ladder, or on the 2nd floor of a building, and the sun will set ever so slightly later. And you'll be able to see a mile or two further away.
If you want a really good test of this, wait a few months and go to a large frozen lake in northern Minnesota. There are plenty of them larger than 3 miles across. You won't have waves to 'obstruct the view'. Just a flat sheet of ice (all but for the curvature of the earth, of course! ;-)