This is why everyone thinking that water suspension will allow humongous accelerations is deadly mistaken. Being underwater at 1km IS NOT THE SAME as accelerating at 100G in a water bath. Not the same! At 1km you have 100atm of pressure on all sides. You can probably scuba dive at 1km just fine if you descend slowly and prebreathe the right gas mixture. Sitting in a 1m tall chair in a water tank at 100G, your head is at 1atm and your feet are at 100atm. This is different! You'll be crushed like a bug. Filling your lungs and other body cavities with water or perfluorocarbons or whatever will not help!

A water tank should instead be thought of as the most comfortable acceleration couch possible, that supports every part of your body with the highest softness possible and no pinch points. You'll also do best if you lie perfectly flat horizontally, instead of sitting or standing.