Congratulations on you recent retirement. There's nothing like it. When you say waterproof, will you be entering the water, or just shoreline? If entering the water, and submersing the unit, then I, personally, would consider looking at the Minelab Equinox 700, or 900. Both high performance units while remaining relatively easy to learn for a new user. If just wanting to be waterproof enough to go knee deep, or less, then besides the two units mentioned, you might look at the Minelab Vanqish line. Although not submersible, they have similar performance to the 700/900 and are considerably less expensive in case you find this hobby isn't for you.
There are many other options, but I'm not experienced with them. If I may suggest, go to the manufacturer's website, and download the User Manuals for any units you might be considering. By looking at the User Manuals before making a commitment to purchase, you will have first hand information about them without the biases that everyone has, including me, about their personal choices.
Good luck with whatever you choose, and welcome to, what I believe is, the best hobby there is. At least it has been for me over the last 50+ years...