My dad was diagnoes with elbow bursitis and he woke up this morning and he said his elbow looked like a sack of water. Is it dangerous? And is it the fluid filled bursa?

Yes it is, and this website will explain exactly what is going on in your dad's elbow

Bursitis Symptoms, Treatment, Causes, Types and Diagnosis by MedicineNet.com

YO! I just am healing from one of those. Tell him not to bump it as he moves around. Mine has taken about ten days to heal up.