Popular wood is a pretty soft wood that will rot fairly easily.  I wouldn't think it would be very durable on a boating application.

Just my two cents worth

   And it has a fairly brittle failure.  I doubt anyone would recommend it for spars, so I'm not sure what the original poster is referring to.  It could do ok as a fully coated wood on interior nonloadbearing parts, or maybe in inlays and other decorative bits.  

   I have used Yellow Poplar wood in many small boats (stringers and other parts) and as stock for paddles, but Yellow Poplar (Tulip Tree, Tulip Poplar) is really in the Manolia Family not the Willow Family with the other Poplars and Aspens. Very easy to get on the east cost, very straight grained, and in between the hard woods and soft woods in strength and weight. Maybe a 'soft' hard wood. Still maybe a little too soft for rails... Although some canoe paddles I made from Yellow Poplar have held up to a very large amount of abuse, sooo.....maybe it would make good rails

As far as rotting, my 12' Skin on frame solo canoe is made with oiled Yellow Poplar stringers and no problems in ten years. she does spend her 'off' time hanging from my garage rafters.

But I have no experience with other types of poplars!

Joel

Didn't the late Robb White often use tulip poplar for planking?  Epoxified, of course.  I seem to recall him describing that in some of his pieces in Messing About in Boats.

.....Michael