Is AppleCare for a watch transferable?

You should have the seller contact AppleCare Plan Administration if they want to transfer the plan to you. And it does matter if the plan is fully paid for or on a month to month payment basis.

Also, before you buy this watch, don't be a victim as so many who come here daily are and make sure the seller does everything required in this link to prepare the watch appropriately for you as the next user --> What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support

About buying used wearables, you might want to read this as well --> https://discussions.apple.com/docs/DOC-250001038

It depends on your region and whether it is month-to-month. Take a look at the terms and conditions which you may have received when you purchased.

You're welcome.