It is possible to choose a cabin, though I’m not sure if it shows you all the cabins available. Firstly if you sign in before you search it helps by entering your details, you then only have to enter the second person’s name. Once you choose your type of cabin & press Book it takes you to a page that has a line above it headed Category with a paper/pencil icon at the end.
If you click on the paper/pencil icon you get the option to choose cabin. Click that & it brings up the cabins available in list view or map view.
As I said I’m not confident that it’s ALL available cabins, but it does give some choice