I use captains varnish in high heat and humidity, Mobile, Alabama, comes out fine.
Brush marks are usually from putting varnish on in too large an area and then your
spending too much time trying to brush it out and some your strokes are hitting areas
that are too dry to even out the paint brush marks your putting down.

I only brush on 1.5' feet max of a kayak at a time, cross hatch strokes, work fast...otherwise you'll have brush marks..do not
go back and try to correct anything in the last section you just did..it will show...usually better to let the varnish sit as is.
I use a 4' foam brush.

If you don't have a bright light set up so that you can see the strokes then your going to have missed spots that will show up when it dries.

You can wet sand with 2500 grit and polish it with a car polisher but i think it removes some durability from the varnish. it'll be mirror smooth though.
I did it with captains varnish.

: Help...I have finished the kayak, but varnishing is driving me
: crazy. I have tried everything. I can't get the varnish to lay
: flat. It either leaves brush marks or doesn't gloss in some
: areas. I've tried thinning, also figure the temp was to hot
: (high nineties), took into a garage, better but still leaves
: brush marks, told I was working it to much, but how can you
: leave brush marks or dry spots? Finally took into living room
: & set AC to 76. Still leaves brush marks and dry spots. I've
: sanded this boat a million times and am ready to give up.
: Varnish I have used is Captain's varnish & Helmsman, spar
: urethane. Both companies say they are compatible. Foam brush,
: Natural bristle brush. Please I would just like to get one final
: flat glossy coat, so I can launch the d#m thing! Thanks for any
: advice