Hmm... How about a floating-yet-submerged pipeline?
Water flowing through plastic tubes anchored offshore
It could start small -- say two 12 inch pipes, then more, or larger, pipelines added once the concept was proved.
Why does this work? For one thing, eminent-domain, right-of-way issues pretty much go away. And the problem of structural support turns into keeping pipeline sections from _rising_, rather than falling (caused by the natural bouyancy of the pipeline and its contents)