I pick up AIS transmissions from vessels 40 nm away sometimes. I use a splitter to my mast top whip so I get pretty good receive coverage and, hopefully, this is also true of my transmit. Of course you loose the ability to check on yourself on marinetraffic or what not once you are about 15 nm offshore and loose your cell signal.
BTW my Son is a mate in the merchant marine and tells me he rarely sees sailboats on the ships radar. Always sees AIS though.