Like Dan said, an AT pro would likely be your best bet on a budget. Just a waterproof coil may be OK in some instances such as going through puddles or very shallow narrow streams- but slip once in deeper water and the whole machine in in the drink. Then your detector is likely done for. Same reason I upgraded my machine. I came close a couple times to losing my X-Terra in the water and I would never forgive myself if I ruined a machine that was only a few weeks old.
I don't know what your budget is on a machine, but for around $650 for an all around waterproof machine, that's doing pretty good. You can also look online for used machines as well and maybe find something for much less. The only drawback is like dan also mentioned, if you are planning to use it in places where there is wet salt then you will still have a difficult time unless your machine is either multi frequency, PI, or can be changed to running a high frequency coil.