I realize the reasons for the island, but it will also make achieving your depth more difficult unless the pond is somewhat shaped like a tear drop with the island in the tail. I recommend trying to get Mike Otto to come out to build it, or see if he can recommend anyone in that area (or a recommendation from Bob Lusk). It's doable, just needs some design work and thought.
I already talked to Bob, and will probably have him come out to my property in the next couple of months. If he recommends getting Mike to come out, that is the way I will go. As for the depth, I will be working with about a 215' x 330' area (not including the island), but I may end up changing the desired depth to 25 feet instead, because of the reasons you mentioned. 35 feet may just be more trouble than it is worth, unless I can find a large diameter concrete pipe (or similar) that can be sunk in a certain area, to create a stable "tube" to gain that extra depth in a small section (I have seen this done down to over 100' in a professional dive pool).
Please make sure to consult professionals about the zipline for safety! Highly recomend the books that were mentioned, You can buy them from the main site here. Matter of fact I think you are in prime OTTO/Lusk territory for a consult from the best.
Luckily, my father is an engineer, so he has some friends that are going to calculate everything for the zip line system (distance, angles, cable diameter, etc). And I already have "Just Add Water" on the way, along with a subscription to Pond Boss, so can't wait for those to show up! I will be sure to document EVERYTHING, so I can share it here!!!
