Alcarondas, the Castle of the Sea was the golden-and-black flagship of Ar-Pharazôn, the last King of Númenor. In it, he sailed to the shores of Aman to challenge the Valar, leading to the Downfall of his kingdom.
The name is not easily translated, but it obviously doesn't mean "Castle of the Sea". It contains alcar "glory", and no recognizable element for the word "sea" which in Quenya is eär.
It was also called Aglarrâma in Adûnaic which seems to incorporate the Sindarin word for "glory", aglar, perhaps adapted in Adûnaic.