It seems that in the US
Jet Ski is a trademark, and US dictionaries write it that way: as two words and capitalized.
The single word, uncapitalized form is acknowledged by Wiktionary and the Macquarie dictionary. This variant seems to be much more prevalent in Australia and New Zealand than elsewhere, but it does occasionally appear in American and British publications.
The word can be a verb as well as a noun. For example, in a review of Prince Harry's book in the
Evening Standard in January:
Indeed after his magical week in Botswana with Meghan, he heads off on a multi-phase lads trip complete with jetskiing in crocodile infested waters, bush cocktails and African beer, and “certain controlled substances”.
Jetski,
jetskiing and
jetskied will be accepted as rare words.
