Tracing a very small leak

[zeehag]

have you checked your water tanks and hoses?
fresh water...water tanks or rain or sink drains. shower drains. leaky deck connections.
enjoy your searching. at least fresh water doesnot sink boat. may need to reweld any steel tankages. could be your drains for sinks as you drain out water some sneaks out. could be a nice lil sneaky O ring at the deck level.
if it occurs when not using drain, then tank and hoses. connections or welds.

[georgeg305]

I had a leak on the rudder shaft of my Catalina 30. The wormhole only leaked when the boat was underway. I.E. the leak was above the waterline when the boat was still. Underway the boat squats in the water a bit and the leak started.
As mentioned powder and tissue paper are great aids.