T i m wrote
Chris Hogg wrote

The big problem with all these floating devices is making them robust
enough to withstand the extremely aggressive conditions presented by
a salt water environment, and more particularly, the winter storms.

Watching programs about the early Engineers who built our
bridges and ships are often fraught with mishap and failure (cast
iron bridges fracturing and trains plunging into the sea etc).

And still manage to do that occasionally like that footbridge in London that
never wasn't viable initially and the bridges that fall down when being
built.

However, it seem most of these things were simply improved upon until
they found something that worked and then lasted many decades.

Not always, see above.

Is it now we have done most of the important / interesting stuff

Nope, most obviously with aircraft.

and now we are left dabbling with these 'alternative energy'
schemes that in most cases seem doomed to fail?

We appear to be into that **** because some like
harry are too stupid to even notice that nukes are
the only viable approach with a country wide grid.

With a real tendency for some engineers to keep playing
with stuff that isnt ever going to be very viable, because
what it used is so variable with wind and solar.

Yes, solar does work very well for the lower power situations
where using the grid isnt economically viable, but that is a
separate matter to how much sense it makes for a national grid.

Is it that we can (generally) send rovers to distant planets,
build bridges that are miles high and long or build very
tall buildings on sand ... but we can't make something
float in the sea and not have it sink or get swept away?

Nope, we do that fine too. The problem is the ****ed
economics compared with nukes when its on the grid.

Is there really no 'will' to do these things,

Nothing to do with will, the problem is the ****ed economics.

like someone tasked with building perpetual motion machine,
especially if there isn't some generous subsidy it seems. ;-(