If someone bought me an Apple Watch as a gift, can they take it back?

In general, an unconditional gift is irrevocable.  There are some exceptions, the only one of which which seems like it might apply is if the gift is given in contemplation of marriage.  Assuming that this was not the case here, and the gift was not conditioned (e.g. he didn't say "you can use this watch as long as we're together", or something like that), it is yours to keep.