You can walk the radio over to a neighbor in the marina or the next guy at the ramp and ask if you can test your radio on his/her antenna.
Rig the radio with a cheap auto barn cigarette lighter plug so you can plug it right in- most boats have the receptacles. Then if the radio acts the same, it's not the antenna.
Make sure your antenna's connector is properly terminated with both signal and shields connected.
Alternately, you can take the radio as is into your local marine store, or West, and ask if you can do the same on their displays.
I expect they will let you and even do it for you, gratis, since it's an opportunity to sell you something. They even have watt meters that can show the transmit power, etc. I'm sure the Mom and Poppers will do it, if you are on Long Island, I have two to recommend.