2. That's cool if you have percise to the ounce weighing instrument. Try weighing yourself before soaking 1 hour in a tub of plain water, then after, to see osmotic or dermal absorption. I read one half to one cup of water, 4 to 8 ounces can be dermally absorbed in one hour submerged, up to neck of course.
3. Maybe the body only absorbs water when it is needed. Maybe you were well enough hydrated before bath?
4. The full moon, prior and just after is when water weight is retained. Full moon in April was on the 6th. At new moon water is low ebb, new moon is April 21st.