I'm 6'1" 205# and ride a 5'11 retro fish that is pretty darn fat and wide. I have no problem sinking it and duck diving is no problem. I am however kinda big across the chest and arms so I have a lot of force up top to do it. Duck diving is more timing and method then it is shear strength so you should be able to dive any board within reason as long as it is sized appropriately. I love my fish by the way and have a hard time putting it down even when the conditions call for something else. Good luck!
Thanks bro, its something i should probably work on. I think im leaning towards a summer fish, slightly better for carving.