If you have a general idea of where it's located I would just start the exploratory digging process. Hand dig a trench perpendicular to the water line about 8 inches deep. Then use an excavator to dig a bigger hole to that depth. Then hand dig a trench in the bigger hole about 8 inches deep again. Then excavator. Add you get deeper you'll get wider. If you get below the known depth then the perpendicular trench gets longer. Take it slow.
Otherwise if you don't want to do that I'd feed as much of a spool of clean copper wire, that I could, into the pipe and have a professional locater service clip onto it and they should be able to locate. If fish tape as stated above.
Look around for a tracer wire it could be hidden somewhere around the pipe terminations. I doubt the installer installed a tracer wire, this is a must for people with experience, but people who just do it themselves or have their neighbor help, rarely include this.