always have it attached to you somehow, via landyard or something.
As for filters, it depends on the type of water you will dive. if the water is very blue/cyan then take a red filter, if the water is more of a green, take a magenta filter. it does help but it isn't 'perfect'. If you have a good editing software, you can usually do a decent job of color correcting even if you didn't use a filter to help.
also, pack extra batteries. i usually have a dead batter by the end of the first dive, and need to switch it out before the next dive.