Well, could be your VHF
too. There is no straight/obvious answer. You should approach the problem in parts, and I suggest going from down to top (your mast). First, check your antenna connection for some "resistive load", so in case your antenna circuit is open, you don't fry your VHF
output, during the diagnose. Best would be a SWR, but a patch antenna would help you eliminate your radio
guess, or bringing it to a friend's boat. Unfortunately, majority of problems in VHF
resides on the top of your mast ....