I was always taught that on the water it doesn't make as much difference as on land
the line of sight is how these short wave lengths work..you can get some skip over the e layer of the atmosphere but not a lot and not dependable
so, The maximum theoretical distance that the two boats can communicate is 9.1 miles (4.2mi + 5mi)
and The maximum theoretical distance the distant boat can communicate with the tower is 17 miles (12mi + 5mi)
unless the waves are 15 feet high I would go with the longer great gain antennas..if you want to talk to airplanes then the short low gain antennas will shoot the signal high....I have been a ham radio operator for over 20 year, so I have some idea of what I am talking about.....got my license back when you had to be able to build a transceiver to pass the course ..and morse code