Hi lauza....Be very careful if booking with OTB, the internet prices advertised are sometimes misleading and you may find not as cheap as they first appear. Just double check everything and ask questions before you commit to a booking.
October is still busy and everything is open, as it is an all season resort, but not sure about the water parks etc.
The Servigroup hotel chain IMO are the best ie Calypso, Orange, Venus and Pueblo.
The Rio Park hotel is a good 3star family hotel also The Regente.
Hope this helps...you will love Benidorm if you like a busy atmosphere.