acclimated vs. acclimatised

According to my research, acclimate, acclimatise, and acclimatize all mean the same thing. These are just regional differences. In the US, I mostly hear "acclimate," and it's always in the passive form, "be acclimated."

"...acclimate, acclimatise, and acclimatize all mean the same thing."
I completely agree. There are plenty of such similar words in English, and we should be prepared to understand all of them. Some of the differences are very slight, such as in the case of "judgement" and "judgment".
Furthermore, I have read that for reasonably comprehensive dictionaries, you need 100,000 words in French, but you need 200,000 words in German, and you need 300,000 words in English!
Just give your acknowledgment/ acknowledgement to these facts. English is a very rich language.

Acclimate is an older word, apparently, held onto by Americans while in Britain, Australia and New Zealand (etc) we moved on and acclimatise has long been preferred. Think too of Fall And Autumn - in Britain we stopped saying Fall centuries ago, but Americans stuck with it. To my ears, acclimate sounds clunky, but that could be because I'm more used to the British English version. Also, aesthetically, acclimatise sounds more agreeable to my ears.