You'd only need to rerun the lights if the cord was cut too short. The lights in their niches will often have a few feet of extra cord wrapped around the light housing so you can pull the lights from the pool. Perhaps there's enough cord so that you can simply cut the PVC conduit and add some more to raise the J-box up above water level. If the person that installed the lights cut the cord too short, then the only way to fix it is to get new lights. Whoever did that installation did not do it correctly. It's definitely a DIY project if you're comfortable working with electrical wiring but, if you're not, then have a professional do it right. Whatever happens with the lights, that J-box is way too low to the ground and needs to be raised probably by a good foot or two to be code compliant.