Providing water is important for your pet rat, as well as feeding them properly. What is most convenient and what most people do is just get an appropriate type of water bottle that hangs on the cage that they can provide to allow the rat to drink water at will, or to be able to drink as much water as they need throughout the day. It's really important as well, in some rats, to make sure that they will accept and drink from the water bottle, they will all learn to use a bottle, but some rats actually prefer water bowls and prefer to have dishes as well. In some instances, it's recommended just to have both.
There isn't really a concern with a rat having access to water as long as it's freely available. What tends to be really important is that you make sure that your rat is not over consuming water. The over consumption of water in a lot of rats could indicate serious medical problems like kidney disease, and it's really important that you're aware of how much they generally do need to drink so that you're paying attention to how many times you're refilling that water bottle.
The other thing that's really important is that you keep the water bottle very clean. Sometimes rats will backwash when they drink from them. The water bottles with stagnant water in them can become pretty dirty, so it's important that you keep the water bottle or water bowls very clean in your enclosure.
If you choose to offer only a bowl and not a bottle, understand that they may get their bedding in it, they may get food in it. The water can become contaminated and get pretty dirty, and this is why it's recommended to use a water bottle hanging on the cage as well, as the primary source of water.
Rats will randomly or intermittently drink throughout the day depending on the type of diet they're on. There are mathematical calculations and ways to determine the average amount of water that a rat needs, but a healthy adult rat generally will be able to get water as needed as long as it's always available, and the only concern really would be if you notice that your pet rat is excessively drinking water, then you'll want to seek veterinary advice.
So overall it's important to keep an appropriate size water bottle. Keep it very clean. Keep it full. In some cases offer a bowl and make sure that your rat is drinking water normally.