What should I get my boyfriend for Christmas?

It's a little hard to suggest things if we don't know his interests.
I'll just throw a few general ideas at you in case any stick:
An engraved watch / pocket watch
A leather bound notebook and parker pen
An experience day voucher
Name a star after him
Something autographed by his fave actor / singer / sports person / author
A suit and tie
Tickets to a concert / show / match / weekend away
A musical instrument
A VR headset / games console / steam deck
A new phone
A computer game
Chocolates
Alcohol
An electric back massager
A book
A leather briefcase
A collectable item
Aftershave
A bonsai tree
Cufflinks
A beard grooming kit
A beer glass
Headphones
A karaoke machine
A drone
A toastie maker
A tool kit
A USB mini fridge

Is he a foodie? Do a small box or hamper with sauces, chutneys, jams (maybe local produce, like from a farm shop), herbs and spices, cheesy nibbles, biscuits, tea, coffee.

Context in the title :0

I think getting him something related to one of his hobbies would be a good idea. But also, I guess just spending part of the day with him (assuming you aren't busy with Christmas celebrations in your family) would be a nice thing to do too!
If you want some general ideas, here are a few:
Lego
A book
Plushies (if he likes them)
A watch/pocket watch
Chocolate/food items
A computer game
Gift cards
Also deciding with your gut feeling is always a safe option.
Good luck finding him something!

He’s a large marine biology enthusiast 🤔

Relating it to myself for a moment, I'm aiming to go into entomology and I'd love stuff like unique magnifying glasses or other interesting entomology tools, rare preserved insects, those kinds of things. With that in mind, I'd say if you wanted to get him something related to marine biology, maybe he'd like things like shark teeth or other preserved biotic marine material (ethically sourced, of course, make sure you're not giving him the marine equivalent of elephant tusks lol). That would depend on him as a person, though.
That being said, don't just get him any old junk that will just sit on a shelf without his interest. Maybe something to do with his favourite animal or something along those lines, but again, that might still be kind of boring depending on him.
I'd say equipment is an amazing choice (calling back to mentioning magnifying glasses for myself) but I figure that would be harder to decide for marine biology. If you have the budget, what about organising a day out or weekend away or something like that? Not sure what would be available to you in winter, but if seasonality is still permitting, something like whale watching could be really cool.