For my 2p's worth
Where Manos has mouted the antennas might be good for cable routing but ALL antennas MUST be mouted away from conductive items, which includes items which hold water as salt water is conductive. (dont forget people are maily salt water .
To work correctly and idea siuitaion would be where no conductive items are within a wavelength of the antenna, on the same level, about 1.9 meters for VHF and about 25cm for GPS. If items are placed closer than this they tend to de-tune the antenna making it a poor antenna. This leads to poor GPS fixes, shorter VHF range etc.
This can be hard to obtain but most of the loss if when you go below half these values, a half wale length.
The other key thing mainly for VHF is height. The higher an antenna the better the reception.
Mounting on a pole is good, but you must get a antenna that does not need a groundplane, which most do, if you going to pole mount it.
If i didnt have a A frame i would go for a transom mounted pole about 3-4 foot high and go for a antenna with about 3dBi gain (the ones about 3-4 foot long).
A good check but not always 100% is to use a SWR meter the reflected power should be 1.5:1 or less.
This is only a guide as a dummy load with give you 1:1 but will not radiate much !!.
Also Phil, there should no reason not to shorten a coax feed antenna as long as you make good job of fitting the coax plug.
Anyone wanting more info on specific instalation drop me a PM