[SOT] Whale Habitats?

Hello,

Lurking of course, since my scientific contribution is about nil. I was hoping I can call upon this list for help with a part of fictional role playing setting I'm working on--not for profit. I was wandering if a space settlement cylinder would work for cetaceans. The premise being a sanctuary or 'ark' of sorts for different species of whales with the decline of the ecosphere of the oceans on earth. Already have a way to get them up there in the setting, just questions of details of the cylinder, which is tentatively named 'Cachalot' after Alan Dean Foster's book about a world given over to the whales.

Supposing that such a project could be done, how deep would the water have to be and would the rotational rate be the same as a standard island three type cylinder? Would 40 kilometer length be considered too small? Would it be like the 'Africa class' sized cylinder mentioned in High Frontier (I think)?

Hopefully this is not too off topic as it deals with fiction, but I want to try and keep it as much with scientific plausibility as possible.

Thanks in advance,

Jeff