I'm a firm believer in the synthetic materials for Paddling trips.
Merino wool is the one exception.

Yes, cotton will work for mid-summer trips, but cotton is heavier than a synthetic t-shirt or synthetic/cotton blend. Takes longer to dry IF you get it soaked. Remember even in the summer, with the right conditions, you can still get hypothermia.

Synthetics compress smaller, weigh less, and like wool, don't retain body odor. They launder easier as well. I take a waterproof stuff sack, put a gallon of water in, and sit on rock in full sun for a couple of hours.
A few drops of all purpose detergent, add clothes, let soak a few minutes and then swing bag around your head a few times, making like a Maytag washer ;D  Dump soapy water in woods, wring and repeat for rise cycle. Hang synthetics on branches in sun or on warm rocks and they're dry in minutes. I've even wrung t-shirts out, and let my body heat dry them .

Cabelas sells a synthetic/cotton blend for those who like the cottony feel. Best of both worlds.

I take 2 complete outfits, plus 3 pairs of socks/ t-shirts/ and underwear, although I might quit taking the underwear. Much more comfortable without, and what's my odds of getting into an auto accident in the woods ;)