Exactly what I was goine to say (what Raven said), plus who would have to look after it???
I guess they might have naively thought that 'everyone' would love a pool so it wouldn't be a prob.
I would question it too, both through the RE (ask them to provide you with the laws in writing) and the TT. Are they willing to drop the rent if you allow them to continue? Cos it sure ain't what you signed for!!!

eta found this though
under the Residential Tenancies Act 1987 No 26

22 Tenant’s right to quiet enjoyment

(1) It is a term of every residential tenancy agreement that:
(a) the tenant shall have quiet enjoyment of the residential premises without interruption by the landlord or any person claiming by, through or under the landlord or having superior title (for example, a head landlord) to that of the landlord, and
(b) the landlord or the landlord’s agent shall not interfere, or cause or permit any interference, with the reasonable peace, comfort or privacy of the tenant in using the residential premises.
(2) A landlord or a landlord’s agent under a residential tenancy agreement shall not, during the currency of the agreement, contravene or fail to comply with subsection (1).