Sometimes water can slow the first stage of labour down. If you feel that may be happening, you can get out again! There is no rule saying you must stay in the pool once you are in it! In fact, it can be beneficial to get in and out of the pool to help the baby move down well into your pelvis.
Some women and birthing people find that the water makes them too hot during labour so they want to get back out again. There is debate about the best temperature for birth pool water to be at but generally, a comfortable temperature for you is best while you are labouring. Your midwives may want to change the temperature to be closer to body temperature as the birth approaches - that is for you to discuss with them.
Using a pool may increase your chance of having a minor tear (or it may not), but it decreases your chance of having a severe tear or episiotomy.
You can read more about the evidence around waterbirth on Evidence Based Birth.