Two years ago a home center when out of business and I bought everything at
80% off.
200ft of 16/3 extension cord, electrical tape, a plug, outdoor box, and GFCI
outlet.
I cut the cable in half and twisted all the insulated conductors together;
so I could use one length as the hot and the other as the neutral. I then
taped them together to act as one cable. Wired one end to the plug and the
other to the outlet.
Seems to work fine. I doubt I will ever run more than 6 amps over it, but
wanted to reduce the voltage drop since it is pretty long. Sadly, 16/3 was
the largest stuff they had left.

Why would I do this? I have a dock at the bottom of a 75' cliff. I don't
have any power there, and carrying a generator up and down 105 stairs when I
need power is unpleasant. Now I can plug the generator in at the top and
run whatever I need at the bottom. Tried it yesterday and it works fine.
The GFCI outlet is probably excessive, since there is already one on the
generator, but at 80% off...

I would like to run a lighting circuit off the outlet to my boat house so I
can put some lights on it. Can I use UF cable for that (it will simply be
exposed to the elements) or must I use do something more elaborate? I will
rarely use it, so I don't want to put a lot of money into it.

Any major safety issues I am ignoring in the whole scheme?

The power company wants $2,000 to run power to a the top of the cliff; the
breaker box and getting it down would all be my responsibility. That, and
the $40 monthly minimum, make my alternative seem reasonable.