As happy as a clam (at high tide)
Translation: счастливый, довольный
Meaning: happy, with the feeling of safety. The idea is that clams are very difficult to find at high tide, so this is quite safe and happy time for them.
Here is a nice explanation of the origin and the meaning of this American expression.
Reason: I came across this in a movie script and got intrigued by the comparison, because a clam is not among associations I"d have with happiness.