The style of machete you're going to want to get depends on the vegetation you have to deal with.
The longer and thinner a machete is, the easier it is to get through lighter materials like grasses and thinner brush. So if you're going to be dealing with a lot of grasses and lighter brushes, an 18" might be a good idea. It also gives a standoff distance for thorny bushes like blackberry, while still working well on thicker stuff (though if it's built
too lightly it might have trouble getting through thicker branches).
For thicker brush and trees, a shorter machete would be fine, I think, so long as it's built fairly heavily. I have a Kabar kukri machete that chops almost as well as an axe; it's about 12" and fairly heavy. But it has some problems going through very thin stuff; not enough speed at the tip, and it ends up just knocking it aside.
A decent 18" machete actually doesn't weigh a whole lot or take up that much space. Unless your kayak is super small, I'd imagine you could find somewhere to stash it.
Take a look at Tramontina and Ontario, plus the Cold Steel Latin machetes aren't bad.
Any of those will be decent but maybe little rough out of the box. Perfect for working grade tools though.
Haven't heard of Tramontina, but I've heard lots of good things about Ontario machetes. Cold Steel machetes usually don't come sharpened from what I've heard, so while they're decent you have to put a bit more effort into them.