I've always thought Unsinkable II would be a good boat name

It's fun but I reckon it's unlucky, Poseidon has zero sense of humour about these things. No doubt he would see fit to determine if you are brave enough to name Unsinkable III.

Sea superstitions are in the category of things I know are completely irrational but I've been thoroughly confirmation-biased by events so I follow them anyway. I know an unfortunate sailor who renamed their vessel and she fell out of the crane at the start of the season, cracking the hull. Last time I was on a charter boat with some mates a crew member made a joke about Poseidon, only for us to find the next day the charter company had hardwired the engine and house batteries in parallel, damaging the latter and causing the anchor windlass to fail at the critical moment. Not only that, I've owned two of those 'sinking ship' glasses despite considering them blatantly unlucky and both times they've shattered in my hand. I know all of this is explained by mundane means, but one can never be too careful.

People think lots of things are good boat names. There's no reason there couldn't have more than one Unsinkable that hadn't sunk, and most boats aren't retired by sinking, so Unsinkable II would fit right in.