Given the cool, damp weather we experienced this spring, anthracnose was a very prevalent and common problem.  Typically, anthracnose is not lethal to trees that are capable of re-foliating.  Also, even though pin oaks will tolerate 'wet feet' more easily than many other trees, It would be appropriate to divert some water away from the tree,  The tree is obviously mature and unless the grade of the landscape has been changed quite recently, the tree has probably had to deal with lots of water for some time.  It is always appropriate to remove dead limbs from trees, especially oaks.  Oak trees tend to self prune and falling limbs can be hazardous.
LS