I landed a Saratoga 40 with 10 oz water (measured) in one leaky float. I thought I had lost one aileron and got her back in. It flew amazingly well for what was happening. I added a drain plug (that's when I had the opportunity to pour out the trapped water inside).
Worse was my G-44 Widgeon. Already a very heavy model (twin engine fiberglass fuselage. I was showing of the differentle throttle fr water steering and, when I eventually took off it went nose up and then fell out of the air.
I'd left the drain plug out. Water came in and all went to the tail when I lifted off. Ugly.