As you say, even if a patient is allowed to drink and has a jug of water, often it is not possible for them to pour it out & drink unaided.
I can see staff want patients to do what they can do to aid their recovery.
For example, in PRH , where I was recently, patients were offered a 'bendy' straw by staff if it made it easier for them to drink .
However, for some patients (such as your dad) they really need a fellow human's help to drink and it is unacceptable if they do not have it.
Which hospital is he in?
I suggest you send in a written complaint on his behalf (not necessarily now).
Usually if you do write to complain the hospital will ask if a verbal complaint was made at the time so probably a good idea to (politely) do this soon.
After the Stafford Enquiry things seemed , from my experience, to improve as far as eg patients' access to water is concerned, if this is not present it makes you wonder is anything else going wrong?
Do hope your father recovers very well ASAP.