I’m grateful but I’m also allowed to be disappointed
It’s not that you aren’t allowed to be disappointed. No one can allow you to feel one way or another except for you. The point is that it’s not a particularly useful or reasonable feeling to keep hold of and focus on. And that is within your control.
You agreed to a surprise but didn’t really want one. He seems to have listened to what you have told him in some ways, just not the bits you wanted him to - he’s heard the city you’ve mentioned and the surprise bit. His surprise is doing something a bit different. (Maybe he would have really liked it if you’d switched things up a bit for the holiday you planned for him. But was gracious about getting another holiday in the same vein as normal).
There are aspects of cruising that could be seen as similar to a beach holiday. Cocktails and lounging about. Does the boat have a spa? It sounds like a short trip leaving from the UK, so I presume you won’t be getting the heat you crave, but could you (and would you enjoy) book up some spa treatments and spend time in the sauna/hot tub to get some of the heat you crave? Try out the restaurants. Dress up for dinner if you enjoy that.
So focus on the being grateful, not on the disappointment. See if you can find his excitement in the idea. Because, yes, as you’ve realised, it would be unreasonable to tell him.
There’s no reason it can’t be a good holiday even if it’s not the holiday of your dreams (which it sounds like you might sort of done for his birthday?). But you will go on lots more holidays. There will be time to feel the hot air as you step off the plane. There will be time to sit on a lot more beaches. For now, see if he’s recognised something in you that will respond well to a cruise, it’s worth a bit of risk. It’s not the end of the world. You might actually be surprised!
And if not, next time, don’t agree to surprises! At least you’ll have learnt that, which is a good lesson in a marriage.