This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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adj. Of a ship that cannot be sunk mainly due to fortified walls and clever engineering.

adj. incapable of being sunk

"The Titanic was not called "unsinkable" - without qualificatio - until AFTER she sank, when the VP stated "I thought her unsinkable...""

unthinkable   

psycho-therapeutic    resource-intensive    dissolved    cargo/troop    maintenance-free