The bad thing here is that we are looking to say "mobile are better anf you VHF"
The cases Codders mentioned are all made to look good and news worthy, how many times do you see " boater saved by using his VHF"? Hardly ever cos its not news worthy.
In practice I carry both and both are always on me. Waterproof VHF, mobile in an Aquapack. Belt and Braces all the time for me but would still go for the VHF everytime first. Why? cos everone in range cah hear you cries for help but hey you all know that cos it was covered well on your SRC course?
I would like to see a proper test to see which works better when you are 3-4 miles off shore in the water and trying to make contact with the shore.
IMHO I think the VHF would out perfome the mobile.