Babies are different
Some are quite active and others may not be so.
At 5-6 months, there is still plenty of room in there so at times the baby's movement underwater may not be noticeable to you--baby not touching the uterine wall hard enough. As baby grows and there is less room, movement will be touching the wall more and you will notice more. So, be patient..Another month or 2. Congrats and good luck.