Won't give any brand specific advice, you'll get plenty. I'd stick within your budget, don't let people spend your money, or put you in debt. It's a hobby, have fun with it. You won't likely get rich, a better machine, might not find you any better treasure. Locations you hunt, a little luck, and how much time you spend, are what pays off.
Pretty much every search coil, for any brand and model, will be waterproof, they are potted in epoxy resin, to protect the wire and shape. Probably a rare exception out there, but never heard of one. You can get away with putting the coil in water, just not the control box. Most shafts are hollow, and can fill with water, and can travel up to the control box, if you set the machine down flat on the ground.
The detector is just one piece of equipment, but you'll need a digging tool, to recover targets. An 11 inch garden spade, usually gets you unwanted attention. Most of us use small, hand digging tools. Walmart sells an $8 Fiskars sod knife, which has worked pretty well for me. You can learn to pinpoint a target, with any detector, but a handheld unit is a good idea. Harbor Freight has a Centech pinpointer, under $20. Watch their coupons around holidays, can be had for $13. It's not fancy, not very rugged, takes a while to learn, but useful, cheap, easy to fix, modify. A hand pinpointer is good, helps you after the hole is dug. There is some sharp, nasty things in the ground, gloves on offer so much protection. Just a little safer, than feeling around clumps of dirt. You should also consider something to put you treasures and trash in, as you hunt. It's best to hang on to everything you dig, toss the trash items in the can, or save for the scrap yard. If you leave the junk, next time you hunt that spot, you keep having to deal with the same trash. You won't get every good target, on one hunt. Some places are replenished, with fresh stuff as well.
Starting off with an inexpensive model, will still get you lots of good stuff, and you learn the hobby, whether or not you want to stick with it, what sort of places you like to hunt most. Then you can think about what sort of features would be of the most use, in your next, and better machine. It's really more person preference, and the type of ground you'll be hunting. What I'm using, might not be what's best for you, same with most of the suggestions you'll get. Just keep it basic, minimal, get a feel for the hobby, grow from there.
