I'm a swimming teacher. Just because you are fit and can cycle through the night or whatever don't expect to just jump in the pool and hammer up and down easily. It takes time. Slow down. Most of us float so match your effort to your ability to breathe. Water skills are transferable so try some backstroke.
If you are serious about improving your skills, most pools run adult improver lessons.
For those of you with strong cyclists' legs, those big muscles can suck the oxygen out of your blood if you kick hard. Slow your kick down; try and do less, not more.
If you're bored, count your strokes and try increasing the pressure on the water with your hands to reduce the number of strokes needed.
Good luck and enjoy.