You may well be hearing a harmonic of their transmissions What receiver do you use? FM on ships is restricted to 10 watts (well it was when I sailed a yacht!) and is from 156Mhz to 157Mhz so pretty near our. Also they don't work on the same frequency, it is split with one tx on one and rx on another, so you shouldn't be able to hear the whole conversation on say S20.
However it is well know that our frequencies are abused by many trawlers. I found a net of Northern Irish trawlers on the 5Mhz band frequency which is a French Navy frequency. I thought that a bit dicey, knowing what the French are capable of. :-) and just below 3.5Mhz is thick with illegal transmissions from all over Europe, especially Italy.
For peace of minf you can get a Band Pass Filter which will greatly reduce any harmonics coming into your RX and also leaving your TX. Very worthwhile. It covers 144 - 146