SD cards are an interesting one. Size is chosen depending how much footage you want on it and the bitrate at which you are shooting. With a 24 Mbps recording bit rate you can store nearly 3 hours on a 32 GB. Unless you film the whole dive that does not happen this means several days of recording. On the other hand 8GB is less than 45 minutes at the same rate so maybe a bit too little however if you don't have ways to back up when you are on a trip is useful to actually put away the card and start a new one.
Another misconception is the speed, to date no camera records at more than 28 Mbps this is just 3.5 Mb/s so a class 4 is sufficient for recording video (not recommended at all instead for stills). However when you then have to read the files to copy them somewhere else is useful to have the highest speed possible a class 10 is 2.5x a class 4.
But you don't actually need a class 4 for recording. If you take stills though you really need a class 10 if you save in RAW as your still frame will be more than 10 Mb and you don't want to wait seconds to take the next shot
So for video only minimum class 4 for still class 10 or higher, there are class 10 that do 20 Mb/s
I have tried Sandisk, Integral, Trascend, Computerbay. Integral and Computerbay have failed me.
Build quality of the plastic is also important so I recommend not saving few dollars to have future disappointments and go for the highest quality
