The big problem is "has a map similar to google maps". The inexpensive handheld GPSs don't have the ability to load or display detailed maps and the maps themselves would eat up most of your budget.
A road map will not be very useful other than to get to the park or other location where the geocache is. A topo map could be a little useful but most cache hunters do not use them (unless they have them for off road hiking).
All the outdoors units allow you to enter the location via the keys, but it is much easier to use software to do it via the computer.
All will be water resistant. The etrex family is very durable. Mine has been dropped a few times (while inline skating, onto roads at speed) and dunked under water with no problems.
Easy to use is a matter of opinion. I use an etrex and with some experiance you can use it with one hand quite quickly. I have a friend that uses her GPS a lot less and loves the touch screen on the Oregon, which I find clunky.
Generally Geocachers just punch in the cache location (easier if you link the GPS to a computer with software) and use the pointer to head towards the cache. For doing that, the etrex H will do well for caching. The H means that it has the higher sensitivity receiver that works better than the non-H etrex's when you have trees over you.
The drawbacks of the etrex H are:
It uses a serial port (you need to buy a special cable and many computers no longer have a serial port) if you want to transfer points or your wanderings to/from a computer. The units with a USB port can transfer to Garmin's Mapsource software for that.
It cannot have any maps loaded.
You need at least a Legend H for those features. I actually recommend the HCx versions since they can record longer tracks of where you have been (I save detailed tracks of the various trails I skate on).
See https://buy.garmin.com/shop/shop.do?cID=145 for details on the various models but remember that these are the list prices, not what you can get them for in a store (usually less).
I bought my newer etrex (Vista HCx) from Darrel at http://www.tvnav.com/productlist.htm.
Darrel Goheenm - GPS@tvnav.com - 785-625-3546