1 Corinthians 3:6 (NKJV)
Most people have planted something at some time in their life. One thing that everyone who has planted something knows is that the seed you planted needs to be watered. Watering a seed is not a one-time thing, it is something that is done over a period of time. You don't water the seed you planted just once, it is a continual process that can use a lot of water. If you only plant a seed but you never water it, then the seed you have planted will never grow; without water the seed will die and your efforts will be wasted. A seed that is planted needs water.
Paul preached the Gospel he had received by revelation from God (Galatians 1:11-12), of how we are saved by grace through faith and not through our works (Ephesians 2:8-9). We need to learn about and fully understand these Bible truths, as well as many others, which is why we need to have things repeated and hear them again and again. This is what Paul was talking about in 1 Corinthians 3:6 when he said that he planted and Apollos watered. Paul himself traveled to some of the same places more than once to further help the people he had already taught, he was watering the seeds he had sown.
In 2 Peter 1:12, Peter said he was reminding the people about something, even though they were already established in that truth. There was something that they already knew about, but Peter said it was important for him to remind them of it, Peter was watering the seed that had been planted. Peter, just like Paul, knew that we all need a continual reminder, a continual process of hearing the truth, the watering of the Word of God, to grow and develop in our faith. We need to see to it that we have a regular time of reading and studying God's Word to plant it in our heart and continually water it so that it grows and develops.