what are possible reason for water getting in the engine

Some of the most common causes are a bad head gasket or depending on the vehicle it could be an intake gasket leaking. Some other causes could be a cracked head or a cracked block causing water to get into the engine. If it just a very little bit of water and you are not loosing coolant it could be just condensation. You should have the vehicle pressure tested and depending on the results from that you may need to do a compression test.