Icom IC-603 signsl range problem

I have got Icom IC-603 radio on my boat. There is a problem with signal transmission on certain distance. Everything is ok while in the range about 500m and then signal is lost. I’ve checked antenna connection: socket and plug on both ends, checked wire resistance continuity. Please advice what else I could check before sending radio to pros. Is there any signal range amplifier that can be adjusted? Kasyan