Junction box must be 6" above ground surface level.
Consider where the conduit is to be routed, how much digging is involved and if it goes under under concrete. There will likely have to be entirely new conduit from the light niche to the above ground level connection, and from there to the power source.

Please, be very careful with this. We've had several people killed in just the last year do to amateur-hour pool lighting installations. Changing a light switch is one thing, tinkering with the wiring in a tank of water you are going to immerse your family in is quite another. The special requirements for pool lights is in Article 680 and not aways obvious to even trained electricians. Make sure you hire someone who knows what they are doing.

Randy is right about the deck box. Sounds like you already may have an illegal and dangerous installation.