because led's that are bright enough to illuminate stuff underwater are expencive.
1W leds are only about $3 each.
My 8x8 true colour display using 24 74HC595's will give a good indication of how well artificial PWM works.
I intend to do a tight loop switching them on and off as quickly as possible.
You could use multiple controllers.
The Arduino for controlling the robot and another one for doing PWM.
A better idea than relays is this
A 8 channel IC for high load stuff.