I believe you are over thinking this one. Pine works great. SPF from Lowes/HD works too. Lowes/HD often sell cedar and that is where I get mine. While cedar does make a lighter boat, its not an enormous amount. I have a paddle made entirely out of pine and can not tell the difference in weight between that one and my cedar paddles.
Yah, straight grain is good, but in reality, get the boards with bare minimum knots. A very few tiny knots wont hurt.
scott