A 20-year-old female presents with sudden onset diarrhea for 4 hours. Watery stools, over 10 episodes, large volume, and vomiting 3 times (initially gastric contents, later watery). Exam: temp 35.7°C, conscious but irritable, sunken eyes, dry cracked lips, heart rate 130/min, soft abdomen with no tenderness, BP 46/20 mmHg. The most urgent treatment is: