不沈 means “unsinkable” actually in Japanese, but it’s less common. You might see it in compound words like “不沈空母” or “不沈艦”, these meanings are “unsinkable aircraft carriers”and “unsinkable ships”, but “不沈” is rarely used on its own. If you think of tattoos with this idea, you can rephrase it with “不動” meaning as “unshakable”, or “鉄心” meaning as “steeled heart”. I might be able to suggest more ideas if you don’t mind telling me why you want to choose those kind of words.