F0: A weak insect F1: A weak and small insect(like a fruit fly) F2: Most mosses. Some of the weaker fungi. F3: Lichens. A large bacterial colony F4: Small bacterial colonies F5: Water F6: Carbon Dioxide and similar nonreactive, noncorrosive molecules. F7: Noble Gasses F8: Neutrons F9: Neutrinos
I'd put water as D4. It might look calm, but if you ever go to the sea, or read a newspaper story about a person who drowned, you wouldn't put it as F.