I have a Garmin 48 which is brill, advantage when purchased of angleable aerial which for mounting vertically in its holder is essential. Not available any more but perhaps the new models can be vertical, Garmin's after sales service is the best around. I have it linked to a Yeoman Sport plotter and autohelm.
Yup... The '48 had just about all the good things in a basic GPS:-
Moveable and detachable aerial (can sling it on deck if reception below is a problem)
Fairly decent battery life (and uses AA's)
Pretty much waterproof
Can run from 5V..40V (runs from DC battery on my 24V boat)
Easy user interface
Made and supported (superbly) by Garmin.
I use Garmin GPS76. Excellent features, logs on to more sats than any other I've 8seen. I've had up to 8 sats logged on at once, I think 12 is the max that can be seen from one place. Also has map detail which amongst other things shows all navigation lights for the world in a built in database. I am in New Zealand and have just looked at lights in England on the screen!!
No external antenna but stays logged on down below with no problem. And absolutely waterproof (can be safely fully submerged).
Mine is used plugged into system 12V and uploads waypoints from my SeaPro software with absolute ease. Also downloads latest versions of map software from Garmin website which is, as mentioned in other threads, excellent.
Garmin 48 used for may years wihtout problems, Had a 45 witch went wrong on holiday and quickly replaced by Garmin with the 48 FOC.
Very good company service
Garmin 12 very reliable long battery life and easy to use. Get the combined cigarette lighter adapter/serialport connector to both use from boat power and also link to computer if necessary
I have used a Garmin 12 for the past three seasons. Accurate, reliable and easy to use. Would suggest you get the 12v adaptor - I run mine through a lighter socket - as batteries can fail at the most inconvenient time. Discount price these days probably about £150 although you can get the posher 12XL
Have to agree with StephenW and Burnham_Bob about the Garmin 12. Borrowed a freinds last year to come home from Jersey to east Coast and it was faultless. Not too many frills like some of them and very handy in a pocket. Cheap to buy but used a few batteries. Good luck with your final choice.