According to Toller :
I have no power at my dock, and carrying my generator down 130 stairs isn't
any fun. I have done it once, and probably never will again.
It is over 120ft, so a #12 extension cord would have excessive voltage drop
on a circular saw

A guy is selling a 250 roll of 12/4 wire for $50.

As mentioned, this isn't rated for 120V, and probably won't stand up
to weathering/exterior use either. It'd become lethal within
a year of exposure (if not immediately).

If it's solid conductor, even worse.

Price out 10/2 (with ground) SO (preferably SOW) (stranded/flexible
heavy duty exterior extension cord), and compare with a "contractor's
extension cord" (_fat_ brightly coloured extension cords) in #10.
[These are SOW, but preassembled with plugs/sockets, and are often
somewhat cheaper than "off the roll" wire.]

Chances are you could do it for roughly $75-100. Not cheap, but
the only thing that'll stand up to any extended period of
deployment.

That said:

Ignore the next two paragraphs if this is for more than one project ;-)

I have been known to run a 12A chainsaw on the end of almost 200' of
linked together extension cords (about half #12, rest #14). It
didn't work great, and bogged down quite easily. But, with a gentle
touch, I managed to do what I needed to without blowing anything
up or producing any smoke.

What I'm trying to say is that you _could_ use your circular saw
on the end of a #12 extension cord if you were careful to not
jam the thing (back off at the slightest sign of slowing down),
went very slow, and didn't make a career out of it.
Do most of your cutting _before_ you take the lumber to the dock...
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.