I would say the #1 Gift, hands down, would be cash.

I would get them this: https://www.amazon.com/Diversion-Charmonic-Functions-Authentic-Perfect/dp/B07545JGM9/ And put a tag on it that says, "Unscrew me!"

Inside I would add another note that says, "Since I know a boat is really a hole in the water you throw money into!" along with like a rolled up $100 bill.

Yes, I know it's tacky, but they're likely to have no end of expenses on a boat that it's hard to buy "things" to plan for now. And if they are cruising they might not have easy access to Amazon, or West Marine, or some other place you get them a gift card for.

The hairbrush also makes a great cheap & simple diversion cash safe.