Mate 20 to 30m is deep none of the surf housings will handle that , plus you will need lights at that depth so your housing will need to hold those as well. Ikelight is a great housing but expensive and bulky.
The cannon 7D DSLR camera has a very good HD video in it and this camera will also film far better in low light compared to HD Video cameras , you get the best of both worlds with the 7D , Professional stills and video at under 2k. Plus you change your lenes
That would be my pick for the camera and a housing I use a Essex but only good for 5 to 10m but really light and compact one hand opertaion.
The photo is the housing I'm talking about is in the photo , the camera has a 24-105 lens on it.