What happens if the Durable Towns of Habsburg Monarchy (Livestock Colony) Level 4 are moved into the Ocean? Do they Drown? Does Ocean's Hungry Grasp gain Energy?

What happens if the Durable Towns of Habsburg Monarchy (Livestock Colony) Level 4 are moved into the Ocean? Do they Drown? Does Ocean's Hungry Grasp gain Energy?

Maybe. Drowning is a special form of Destroy, so it works just like a restricted Destroy effect ("2 Damage to those Towns only" per Town moved into the Ocean). It only grants Energy for Invaders successfully destroyed by that Destroy instruction.
E.g., After you move a Town into an Ocean, the Town takes 2 Damage. If that’s enough to Destroy it (because the Ocean has Blight or the Town was already Damaged), then it Drowns normally. Otherwise, it follows the usual rules for Invaders/Dahan that fail to drown in the Ocean: you Push it to a non-Ocean land. If the land it goes to has Blight, it will immediately be Destroyed, but Ocean’s Hungry Grasp will not gain Energy.
If 4 undamaged Towns are pushed into the Ocean with a single Action, that results in “8 Damage to those Towns only”, which you could distribute as you wished. If you used it to immediately Destroy 2 of the Towns, those Towns would be successfully Drowned and grant Energy.