I went riding with a friend of mine not to long ago and we rode her horses. We came upon a VERY dry sandy wash and as we rode we were talking enjoying the day. I noticed my mounts head was VERY low and I can see he was making train tracks in the sand, so I turned to the horse’s owner and asked if he was getting ready to roll and she said no he always does that in sand. No sooner I turned my attention back to the horse and started to tell the owner he is acting like he wants to roll he was down having the best roll of his life. I didn't get to finish my sentence that's how quick it was. I still laugh about that experience.
Ride with some English slip on spurs if you’re not comfortable with slip on spurs get ones with straps. Nothing fancy. Don't use them until you come upon some water. Pay really really close attention to your horse’s actions. Once you recognize the horse has a need to roll move him out and use the crop suggested by BeautifulBay. Move him out with all your energy. Use your seat, your legs, your spurs and your crop. You might want to turn his head back to the direction you came from while using all of your aids. Totally distract him from his goal and move him forward.
Same as above, but with a twist...
Since you know he has this habit. When you see the water come up on the horizon start moving your horse forward with all your aids just as you approach the water and through the water.
Deter him of his goal.
I don't know if this will cure his desire to roll in the water, but as you go through this lesson time and time again he will understand with just a slight touch of the spurs that it’s not time to roll. Of course it will take you awhile for him to learn this, so don't expect miracles. Be patient.
I knew of a horse that had the same problem as yours. He would come upon water and down he'd go saddle and all.