I too had a placental lake with my last pregnancy and all was fine. The following is from babycenter.
"Placental lakes are seen on ultrasounds as black areas on the surface or deeper inside the placenta and are actually pools of blood. There have been one or two cases reported where very large placental lakes were thought to be associated with poor placental function but it is now known that almost all placentas have one or more lakes by the third trimester; they don't seem to affect the baby in any way. A recent study found no association with placental abruption (bleeding from the placenta during pregnancy), high blood pressure or pre-eclampsia, premature labour, small babies or stillbirths."
Hope it helps!