Sadly we suffered a problem last week on ch12.....for about 1.5 hours all we could hear were dogs barking????
Although Coast Guard and VTS still only transmit at 25w, they generally have some pretty big, high gain, low loss 'sticks' to help broadcast the message.
It would be difficult for a leisure boat vhf set to drown out the CG or VTS, especially if they are close to shore. Offshore and you happen to be on the threshhold for range of a CG broadcast station then somebody nearby with an 'open mic' could very well cause you problems.
I'm always dismayed at how many times we hear children messing about with a VHF set sometimes (but luckily rarely) for hours. Bring back electro-shock therapy!