I used Doug fir flooring for stringer and stem/stern plates in my first boat and will never use it again. As you know, it's heavy. More problematic is that it's brittle and tends to shatter when using a router to round edges. It doesn't plane well either, being prone to tearout and splintering.

You shouldn't have trouble finding white pine. Look for C or D select grades. A 1x8 with symetric grain can be ripped down the center of the grain into a pair of gunwale boards, with a bit of wood to spare. A perfect 1x6 could also be used, if you can find one. It's a joy to work with compare to Doug Fir and considerably lighter.

Were you planning to use it for the ribs, too? If you can manage to bend it, I'd like to see that! Oak or ash is far preferable and bends easily. Just make sure that your rib blanks are vertical grain (cut from the edge of a flat grain board).