I am not sure how THEY know specifically but I can tell you this...
Google Ads is now capable of reading your gmail messages (yes, they read your email) as well as watching where you go.
If you have been looking up web sites for tours in the Bahamas, and if you have been to a number of cruise web sites, then Google will "tag" you as a likely cruiser who is going to the Bahamas.
It is sad but true - I remember 8 years ago people were still so concerned about web privacy that California passed a law against spammers. I doubt if even one person has been convicted under that law.
Privacy on the web disappeared almost completely about 6 years ago. Even if you use "anonymous" browsers that delete cookies when you exit, THEY still keep a record of where you went.
I see it most when I get close to actually buying something - for example, I went to a site that sells honeycomb honey, and I actually filled out the shopping cart to order but changed my mind at the last minute. Within 15 minutes I was getting ads on all kinds of web sites offering me a discount coupon for that specific honey shop.
Another time I sent an email (through my gmail acct) to someone asking about his specific model of guitar for sale on Craigslist. Within an hour that same guitar was showing up in ads on web sites for sale at big online music retailers, and this is a specific model guitar, like if you wrote an email about a certain collectable toy doll from the 1950s and then saw an ad for the specific toy doll for sale someplace else.
It is truly scary how much privacy we have given away on the last few years, and there are no laws protecting us from all this snooping. And people thought the NSA was a problem, they have NO idea how much corporations know about all of us.