Wait for one to pass and then swim down, charge
the R1 dash and launch yourself to the left. When you stop moving you'll be
right above one of the safe areas so hold square to swim down there. When the
next one passes hold triangle to swim up, then do the R1 dash, when you stop
do it one more time and swim normally for whatever distance you have left to
cover to make it to the next safe area. After this you can swim normally to
the end and descend down the circular waterway.