"Oci-One Kanubi" wrote in
ups.com:

Convertible pants are good, as are the convertible shirts (roll-up long
sleeves) that you can buy in the same kind of store that sells the
pants -- purveyors of camping and trekking gear, typically. Be sure
they are of synthetic material.

I like shorts and short-sleeve button-front shirts from Columbia
Sportswear and Ex Officio, usually available in the same stores or in
stores that cater to fly-fishermen. These are of quick-drying
synthetic material and I wear 'em with socks and underwear of a similar
material.

Several mail order places carry synthetic shirts like this. I actually
prefer the long sleeves for UV protection. They're often labeled as
"trekking shirts". Sierra Trading Post is a good source for these. LL
Bean, NRS, Cabelas, or one of many other outdoor gear mailing order internet
sites have them as well.

I wear Converse Chuck Taylor's All Stars in my whitewater canoe, but
I'm a decided minority on the river in that respect.

I always carry a paddling jacket in case of rain or other unexpected
chill, and if I'm paddling some stream so tame that I won't be wearing
a helmet, I wear a long-billed fly-fisherman's baseball-style cap --
also of a synthetic material, and with a bill stiffened by something
other than cardboard, so I can dip a capfull of river water to cool my
head, without ruining the cap.

A few years ago I picked up a baseball style cap at Florida Bay Outfitters
that was made of a synthetic material and had a neoprene bill. It's nice
having a cap that'll float if you want to cool off by doing a couple of
rolls.