I use a Garmin GPSMAP 478 as my primary on my own boat and backup on chartered boats. It's small enought to fit in my carry on luggage and very reliable. Every charter boat I've used in the past 10 years has had a GPS chart plotter built in but it is nice to set up potential routes on my computer at home before I get there. I then run my Garmin along side the charter boats GPS (which will have a bigger screen). It is great to have both maps (2 sets of data on 2 different GPS devices) to refer to and to compare.
That said, since you have an iPad, I really like the iSailor app
You can download it for free from the iTunes store. It comes with one free map so you can get a feel to see if you like it. Then, IIRC, you can download the entire caribbean set of maps for $25. Pretty good deal.