Hiya
If you're looking at just a GPS, the the Garmin GPS72 is hard to beat for value for money. Sells for about R2000, which includes mounting bracket and external power cable. I used GPS72's for many years, off quite a few of my boats. Very reliable and very accurate.
The Garmin ETREX is slightly cheaper, but i haven't used one. Seeker1 recommends them, so they will be good!!
If you start looking at plotters, the price starts increasing.
The Garmin GPS76map is regarded as one of the best value for money handheld chart plotters.
Next up is the Garmin GPS60map. Also about the same price as the GPS72map, just a newer generation.
The current top of the range handhled chartplotter is the Garmin Colorado 300 mapping unit. At R6000+, its a pretty fancy toy for fishing purposes!!
If you're looking for a dual purpose unit, ie. a plotter for the boat and a gps for the car, you also have a few options.
The Garmin Nuvi 500 offers this. Waterproof, touch-screen and is designed for both car and boat use. Costs about R5000
The Garmin 276c is another dual purpose unit. Costs around the R6000-7000 mark.
When it comes to chart plotters, you will still need to buy the charts for the unit. This will range from R1000-2500 (depends whether you buy in SA or not), for the Atlantic Blue Chart, and R3000-R4000 for the more advanced G2 Vision cards.
In reality, a standard GPS, like the GPS72 or Etrex will work fine as a fishing instrument. They're all water resistant, so simple pick the one you like best!!
Regards
miles