When control of an At Sea or In Shore box or all-sea hex
changes, enemy units in the box must “retreat” from that box.
From an At Sea box, they may retreat to a friendly, connected
In Shore box or to an At Sea box. If in an In Shore box, they
must retreat to a friendly port in the associated Sea Zone. Units
which can not retreat are removed from play.
7.3.5 Naval Unit Losses
All Naval units are immune from losses.
These two rules appear to be contradictory.
Immune from losses means they can't take losses in combat, etc. Removed from play isn't a loss. I can see the confusion, however.