Water, water everywhere, but not a drop to drink
If you are properly hydrated before you start, and its not a physiological need to drink there isn't much you can do. One thing I would suggest is if you are hydrated but feeling thirsty, in or out of the pool, speak with a doctor as extreme thirst can be a symptom of some serious disorders including diabetes.
If however you are hydrated, and don't normally feel thirsty during other training, I would seriously approach this like any normal training regime. Start where you are now and look to swim for longer periods of time without drinking. Even if its just, 400m, pause no drink, 200m pause drink, building up your time between drinks will help.
With regards to cramping, this again comes down to the source of your thirst and how you manage it. If you are physiologically thirsty, then lack of salt and such is going to be a problem you need to manage. You also need to recognise that you'll be losing less water via sweating through a swim leg than you would over an equivlient length run or cycle.
However, you mentioned that you need to drink every 500m due to feelings of thirst. This is the analogous to someone needing to drink every kilometer of a run. While it may make you more comfortable, I think you'll that unless there is an underlying health issue as mentioned in the first sentence, you should be able to build up to being able to safely swim for 3km in competition as long as you properly hydrate before and after a session.