Still waters erode the shore or Still waters run deep.
More Polish Proverbs:
A guest hammers a nail in the wall even if he stays only one night.Polish
Don't divide the fur on the bear.
Polish
A good appetite needs no sauce.
Polish
Speak well of the dead or not at all.
Polish
There are a thousand paths to every wrong.
Polish
Don't talk about others, they won't talk about you.
Polish