Nothing to fear from Halong Bay..it is an open body of salt water. We did a cruise on our own ( pre-Vietnam, Cambodia, Thailand tour) and there was an opportunity to swim, but it was too chilly in January. And, if anyone bails on that beautiful Four Seasons pool in the Serengeti, it is his or her loss! Furthermore, if brain-eating bacteria is your worry, you should probably stay out of hot tubs in spas!
Yes, but the water temp there and in the pool at Mt Kenya Safari Club were quite brisk!!! Yikes!! : ) Reminds me of a Seinefeld episode. : )