-Off the shelf, or, if you happen to live in a country designed by Slartibartfast, find a fjord.
The view from my office window includes a fjord more than 500m deep; a couple of hours drive to the south, you can get down to 1,300m/4,000ft or so a few hundred meters from the shore.
You’re probably going to have to go off the continental shelf[1] to get 700m it greater depth.
This will make your plan somewhat less trivial.
1. https://en.wikipedia.org/wiki/Continental_shelf