Unless you are really skilled with tools and are willing to do lots of research and learning, and just want the challenge of building an underwater machine...buy a machine made to work in the water.
For real shallow headphone dunking, almost any of them will work. I could name them all and they all work pretty good. Some people like one brand over another, but that's the individual expressing itself rather than the ranking of a machine.
One major difference is do you want to use a PI or a VLF? Both are good in the correct applications but I would say the PI is a little more difficult to master.
The next issue is price. There are some detectors, middle of the price range like Tesoro and Headhunter, that many claim works just as good as the $1200 machines.
A major consideration is reliability. Water machines take a lot worse abuse and are subjected to challenges land detectors are not. Which ones are the best in terms of reliability is debated continuously on detecting forums. You might check out the manufacturer's warranty. Some are one year, others two years, and one is lifetime. Then read how well that manufacturer back their warranty. Read for a couple of hours and you will start to see a pattern.
You build your own pattern. I have built mine and no need to prejudice your independent reasoning. Regards...Jim
