@barnee
Yes, only when Landing Crafts are attacking can they be chosen as casualties.
Example:
United Kingdom attacks German controlled Normandy/Bordeaux - using 6 Landing Crafts.
Normandy/Bordeaux is reinforced with Landmines - 6 mines at the border to sea zone 105 and 6 mines at the border to sea zone 110.
The UK player chooses to attack across the border to sea zone 105.
During this attack 2 Landing Crafts carry no units. In case that some Landing Crafts are struck by mines, these 2 empty Landing Crafts are the first to be taken as casualties.
During the Combat phase, the Landing Crafts that survived the minefield can be chosen as casualties before any other units.
Remember:
If a Landing Craft is struck by a Landmine all units inside this Landing Craft are also lost with this Landing Craft.