I thought I would offer up an alternative to a GPS.

I would consider where I was likely to paddle first.  You may be surprised that a modern smartphone can provide the functionality you need depending on how far afield you travel.  If reliability and distance afield are concerns, go with a traditional GPS (especially if you view this as a critical piece of safety equipment).

I have a really old lowrance gps, but I now use my iPhone almost exclusively paddling.  I use an app called "Avenza Maps" and was able to easily find and load all of the nautical maps for Lake Champlain and USGS maps for the Adirondacks and Vermont.  Make sure you load all of the maps you need at home using wifi.  It GPS locates me on the map, allows me to add waypoints and markers and allows me to measure distance.  Modern smart phones include a GPS as well as positioning via triangulation using cell towers.  This means the iPhone (or most Android phones) will locate you even when there is no cell tower.  Because of this I have yet to be able to not get a location, though the location via GPS alone is less accurate than a good dedicated GPS... it is still more than good enough for my needs.  The full size screen and easy manipulation of a smart phone is way better than any GPS I have used.

   I still use my old Garmin GPSmap 76 and it's all I need.  It's fussy connecting to new computers, but I've figured it out so I can download tracks, upload routes, etc. New Garmin model closest is the GPSMap 78.  Not as many zoomy features as an iPhone, but solid, floats, watertight, and has all the needed nav features baked in.  And, if you lose it for good in the deep blue sea, you are not out a very expensive phone w/ all your data, pix, music, life.

   My wife and I use the Garmin GPSMap 78, it works well for plotting courses, generating tracks and the map data includes depth markings, navigational buoys, and submerged obstacles.  The device eats batteries so we switched to rechargables early on. My wife also uses the Garmin Vivoactive smart watch with GPS enabled, while this device is not very good for mapping a course it easily provides trip data, and uploads the track to the Garmin Connect site and she can easily reference past trips.  

When I do use a GPS, it's my Etrex Legend from 2002. Handheld, waterproof (mostly) and rugged enough that it still works after 15 years. Maybe there's some still on Ebay.

But what I've found is that the more I paddle, the less I've been using the GPS. On the Chesapeake I stay in sight of shore, so memorizing a few landmarks is all I need there. On the tributaries, well a river is pretty much set in its bed, so I'm going either where it's going or where it's been. The Sun tells me the approximate time and/or heading. The size of the bow wave tells me my speed and my effort to achieve that speed tells me if I'm fighting a current or being helped by one. Ditto for wind, as well as feeling it on my face or seeing it in the trees.

So while I really like my Etrex, these days I pretty much stick to my internal nav systems. Waterproof and no batteries needed :-)

Laszlo

Just the other day I used my old Etrex Legend C to navigate the meaners of the Osgood River in the Adirondacks. Runs for 24 hours on two rechargeable AAs and doesn't need a case. Had a quick look at newer models and was surprised at how expensive a replacement would be. 

^ meanders. My old Adirondack Paddlers' Map was pretty out-of-date and not detailed enough.

If you consider an iPhone app check out MapMyRide. It was originally created for cycling (which I use it for) but has been expanded to all forms of transportation and exercise. It won't give you a detailed nautical map but does track your current location on a map showing you time, speed, distance, etc. It gives detailed reports on your trip and the data is uploaded to their web site so you can view your routes on the computer as well.

I haven't tried it for kayaking but hopefully next summer, I'm still in the pre-build research phase.

It is impossible to give a good GPS recommendation without knowing more about how and where you plan to use it.  Here are some general thoughts.

If your need is only for very basic navigation, you should consider a wearable.  Navigation functions are limited but they are very good at recording and downloading your trip.  They are waterproof, easy to use and have good battery life.  The negatives are that they have small displays and a limited number of data fields.     

Basic handheld units are next up the GPS food chain.  They come with a variety of capabilities and features depending on what you want and are willing to spend.  All but the very cheapest will have better processors and more sensitive antennas than wearables, resulting in much better GPS performance in tough conditions.  They have larger displays with numerous data fields that can be customized.  Most come with PC based planning software so you can create waypoints and routes for larger adventures.   Some units will also have a built in compass and barometric altimeter.  For general navigation situational awareness, chart plotting is a wonderful tool but you need to make sure that you have the good map data.  The “base map” data on many units is insufficient for navigation.  Marine charts are good for coastal areas and on some larger lakes.  For more remote areas, topographical maps are a good choice.

Many advocate smart phones/tablets instead of dedicated GPS units, but I am not a fan for other than urban/on-road navigation.  They are generally not waterproof, can be hard to read in direct sunlight and the touch displays do not work well when you have wet fingers.  If you paddle in areas with limited cell coverage, the battery life of a smart phone can be very short because it puts itself in high power mode.  You can solve the battery life problem on some devices by going to airplane mode, but on others airplane mode also turns off GPS.  The GPS receivers built into smart phones and tablets are the lowest capability and the very small antennas are low sensitivity.  In the areas that I paddle with dense tree coverage, my cell frequently loses GPS position.

If you will ever use your GPS for more critical navigation, you should get an augmented GPS.  In North America that means WAAS, but there are other augmentation systems in other parts of the world.  Augmented GPS position accuracy can approach a few centimeters given an unobstructed view of the sky.  A side benefit of this improved accuracy is that it enables the GPS to be much more accurate at computing near real time speed.  It is a math thing, but the accuracy of real time speed info provided by non-augmented GPS units is very poor at kayak/sailboat speeds. 

I have two Garmin wearables that I use on virtually very paddle.  I wear a Garmin Vivoactive HR on my wrist that records my entire paddle including cadence and heartrate.  It downloads through my phone and I can analyze my workout/race using Garmin Connect (see picture below).  I also strap a 310XT on the foredeck so that I can see it without having to stop paddling.  For inland exploration paddling, I have a Garmin Oregon 550T.  It has a detailed topo map and easy to use PC based software for pre-planning my adventure.  For coastal paddling, I have a Garmin GPSMAP78SC which has WAAS and built in marine charts for the entire US.