As the proud owner of an Irwin 37, I would say it just depends on what type of blue water you are thinking about. If you are thinking about within a couple of hundred miles of the coast, or around the Gulf of Mexico or Carribean, an Irwin should do fine.
If your goal is to cross oceans or sail around the world, I would probably look for something more heavy duty. The Irwin might do it, but would need some work to strengthen it up a bit.
In the size you are looking at, either the Irwin 37 or 38 are good choices.