Okay, the summary:
If you're only using it for pinpointing sites (marking them so you can drop on the same spot) but not for general navigation, *any* GPS receiver will be perfectly adequate. While having detailed maps (or marine charts, even) is quite nice, if you can get to the vicinity of the site, all you need is the most basic of GPS receivers.
You may read that GPS receivers with WAAS (Wide-Area Augmentation System) support will give you much tighter fixes. That is certainly true in the US, and down here near the Gulf of Mexico, WAAS performs *beautifully*. Over in Egypt, on the other hand, it is basically useless. WAAS is made up of fixed ground stations, which compute correction factors to be used by WAAS-enabled GPS receivers, and geostationary satellites which relay those corrections down to those WAAS-enabled GPS receivers. Even if you had a clear sky to a WAAS satellite, without a relatively close ground station, you wouldn't have any reliable correction factors. (Note: Europe's WAAS is called EGNOS.)
If you want a GPS receiver that can survive a splash or a quick accidental dunking, you want to look for one certified IPX7 (or IP_7, with a digit in the blank). That designation means that it is rated to survive submersion in one meter of water for 30 minutes, which should cover any accidental wettings. *However*, not all GPS receivers float. My Magellan Meridians float, but my basic yellow Garmin eTrex (among the most budget of handheld GPS receivers) sinks like a rock. If I'm going to use the eTrex on a boat, I'm going to keep it in a sealed dry bag for *flotation*. (Also, be aware that the ratings assume you're not pressing any buttons. If it's wet and you press a button, water is liable to get inside, and it's a pain to take your GPS receiver apart, rinse it with deinonized water, and wait several days for it to dry. A little dry bag will prevent that eventuality.)
Finally, you want a GPS receiver that has a computer interface. Most modern handhelds do, but a few, like the discontinued base model Garmin Geko 101, do not have the capability. You may have to buy the interface cable separately, and if you want nice mapping software, that's extra as well. Basic coordinate uploading/downloading software (such as
EasyGPS) is available at no cost, and the convenience of transferring waypoints to and from a computer is not to be laughed at. (Additionally, there are sources of waypoint data online, and it is much less work to convert and upload them than to tap in each one individually.)
Anyway, that's my commentary. (I've been geocaching since May, 2001, so I have a bit of background with recreational use of handheld GPS receivers. :biggrin