The five day holiday weekend (Saturday April 27th thru Wednesday May 1st) drew a lot of tourists to Đà Nẵng beaches.
Most of these people are Vietnamese citizens taking advantage of a chance to take a long vacation within Việt Nam:
Fortunately for foreigners who are "sun worshippers", these huge crowds are at their peak in the morning (between about 5 AM & 8 AM) and again in the late afternoon and early evening (between about 3:30 PM & 6:30 PM).
Head to the beach between 9 AM & 3 PM and you'll probably find only a few fellow foreigners braving the hot sun rays.