Well God is showing you that He is calling you into the prophetic. Prophesying. The water is the symbol of the Holy Spirit and you being baptized into this new gift. He is showing you that He will show you things very vividly, that’s why you are seeing the colors.
The fact that all of this happens at night may mean that you will go through a very rough period – trials, tribulations and then the outcome will be this gift.
I believe the coral are your angels that you will now have. Coral protects the coastline from waves and they also provide food, this is why you are swimming in the middle away from the edges. He’s assigning you more angels to guide you through this journey you are about to embark upon. Remember Psalm 91 says He will give his angels charge over us so that we will not dash our foot upon the stone.
If you are not already seeing angels you will become aware of them now and start seeing them as well.