Is it safe to drink hot water for heartburn and indigestion or use hot water bottle on tummy during 1st trimester?

Yes..: Although there are other things that may work better. Such as changing your diet to eat less rich, spicy, and fatty foods, eating smaller meals, cutting down on caffeine and alcohol, stop smoking, and elevate the head of your bed with towels or pillows to 30 degrees. You can also try antacids, ranitidine, and Omeprazole if none of these work.

Yes: Neither intervention you mention is associated with danger to the pregnancy or fetus.