On longer dives, let's say over an hour long, the air from the cylinder becomes rather dry and can cause a dry throat - what are possible ways to have a drink when scuba diving? Are there alternatives to drinking?
I have no experience in this but It seems logical that you can drink while underwater.
Most importantly you should use a soft pouch as a container with a straw. This keeps pressure equal and prevents seawater from flowing in.
Dehydration is a very bad thing when diving and can lead to DCS (Decompression Sickness) as the blood thickens.
One of the most important scuba devices is a Pee Valve which allows you to urinate under water when wearing a drysuit. Most importantly it means that you don't deliberately avoid drinking before diving, so you don't enter the water dehydrated.
Whilst it's possible to drink underwater -- using something like a CamelBak bladder or pouch drinks -- you're unlikely to need to drink for recreational-length dives (under 60 mins) if you're not dehydrated in the first place.
Also water's colder than the body, so you won't be sweating. Agreed, with open circuit, you will be breathing out moisture but this doen't appear too much in reality. My dives are normally in the 90 to 120 minute range and I don't feel overly dehydrated when surfacing -- although I do look forward to a cup of tea on the boat! I measure this by the simple fact that my pee valve gets a lot of use even on the long decompression stops.
Extremely long dives, such as extreme depth or caving where dive times of several hours is common, tends to be done using Closed-Circuit Rebreathers (CCR) which means you breathe moist gas. For the longest of these dives an underwater 'habitat' is used where you can pop your head up out of the water to eat or drink -- think big upturned bucket.
Due to anatomy, pee valves are pretty straightforward for males who will simply don a "self-adhering male external catheters" condom/catheter which is glued in place during the dive and plumbed into the pee valve pipe (example; Rochester/Bard Wide Band). These are basically thick condoms that are extremely sticky when rolled on. They have a pipe on the end to which the pee valve pipe is connected. The pee valve itself has a one-way valve to prevent water flowing backwards as the pressure increases with depth.
In use they're great. It's only the first time that feels odd; thereafter it's pure comfort underwater.
Post dive use of medical adhesive remover (such as Apeel spray) makes the removal process painless. Personally I never dive without plumbing in as it completely ruins a dive if one doesn't.
Females have to use other methods such as a "SheWee" device to plumb into or use nappies/diapers.
For very long dives in cold water, such as deep caves, an underwater habitat is used during decompression. Effectively these are like lift bags where you can put your head above the water to eat, drink, talk, etc. Larger habitats enable one to climb out of the water whilst still under pressure, especially important if it's very cold and you've many hours of decompression to complete. For extreme depth dives, >200m, the decompression obligation could be 10 hours or more. With this duration in the water, keeping warm becomes essential.
Cave divers use habitats which are anchored to the bottom and sometimes can even be raised/winched up to match the decompression profile. Flexible, portable habitats are more commonly used than solid ones, simply due to the logistics and costs of getting it in place. Habitats can also be 'floated' against a suitable height cave ceiling, for example a large (airtight and strong) bag which is filled with air from a cylinder.
Depending on conditions, the diver would probably need to continue to breathe from their rebreather as the 'air' in the habitat would contain too much CO2 from occupant's exhaled breath.