Coax cable loss

Antennas receive the signal, receiver makes it understandable. Between these two coax cable carries said signal, which gets more and more tired the further it travels (coax length) and the bumpier the road (coax quality). 
Being tired means you lose energy, so poor signal cannot tell you all the information.
Less info is called signal loss, how much depends on coax cable quality and length, 

To improve a signal you can:

1) Decrease the length of coax cable - less travel, more signal
2) Improve coax cable quality
3) Add a preamplifier - this is like an energy drink to signal

How to calculate signal loss

Coax cable loss increases with frequency. Online resources give this loss in decibels, which is logarithmic, so 3 dB is half the power  - transmitting or receiving makes no difference. 
Decibels and logarithmic stuff is complicated, so measure the

length of coax cable, note the writing on it and google this plus "attenuation". Attenuation means weakening of signal in radio speak. So if writing says RG 6, google "RG 6 attenuation". 
Get a decibel figure with this online calculator. Enter values on top - results in feet will be on the left, meters on the right, you get a figure in decibels.
To understand decibels in a "this is how much percent of info reaches my receiver ", use this online calculator. It simply converts decibel to understandable percentages, with 1 being 100 % of signal received at the antenna.

Enter number to the left of dB with a minus sign, select energy size on the right, then press calculate. 
For example, common RG 58 used at sea in 20 metres 4.4 dB is lost: only 36 percent of signal reaches the receiver.

Decrease the length of coax cable

More length = more loss.

Use the minimum length of coax required between antenna and receiver; excess lengths will not only lose more signal but will also pick up electrical noise.
Finding an antenna location is a compromise, in an urban setting getting away from household electricity, gadget interference and TV radiation can be challenging. 
In the illustration the difference between signal strengths might be small on screen, but that -2 dB means 37 percent of the signal is lost. 

2) Improve coax cable quality

Better coax will lose less signal. Four times less loss is four times the cost, so selecting a cable is entirely up to your budget.
LMR-400 is commonly used, which is the upper limit in terms of cost and flexibility for mortal souls. 
Performance improvents will be only realized with long cable runs, and / or high frequency applications, or if the antenna is used for mission-critical applications e.g. transmission line for life-saving equipment such as mounted VHF on yachts. 
Check the table to see the LMR-400 has much lower dB loss figures - but performance comes at a price. 

Use a preamp

For receive - only applications mount a preamp at the antenna to overcome long lengths of coax cable signal loss. 
Punters seeking signals above 800 MHZ - such as monitoring trunked emergency comms in the States - or ADSB enthusiasts hunting 1090 MHz will need a preamp with cable runs over 30 feet / 10 metres. Most of these require line of sight / unobstructed view of surrounding terrain hence necessitate long cables to mount antenna on top of the house.
Sailing vessels with antenna on top of the mast are better off using a LNA4ALL powered by nav lights as with tall masts and corresponding excessive cable runs signal loss will be horrendous.

Significance in marine applications

Highest point on a vessel, usually mast top is the traditional home of antennas, antenna is normally a stainless steel whip.
Antenna higher up - better range due to increased line of sight. 
Mast height increases with length, Bavaria range as an example, mast heights from waterline in metres:

Cruiser 33 - 14.8 m, Cruiser 37 - 16.8 m, Cruiser 41 - 18.6 m, Cruiser 45 - 20.7 m

33 feet / 11 metres is a popular length for a sailing yacht, use that as an example: to top of mast cable run from navigation station is around 18 metres, to aft rail, cable run is 4 metres.

Cockpit rail mounting enables short lengths of coax to be used, so less signal is lost. And climbing the  mast is not easy...

Commercial marine antennas are normally sold with 20m RG 58 coax cable. I phoned chandleries in Ireland and the UK, RG 58 is the most commonly available "radio cable". 
My deepest respect to the few exceptions, but all sellers I've talked with offering life-saving equipment seem to have no clue about technical aspects of radio. 
Notable exception is Nevada Radio, a well-known ham dealer also selling marine gear, whom I can recommend for excellent and knowledgeable customer service (no affiliation with them).
Signal loss also affects transmitters, using the above example of 20 metres of RG 58 coax, your 25 W mounted VHF becomes a 9W transmitter. 
Note that it is more than enough to establish communications, but results can be improved by using LMR-400.

If you wish to have AIS on board, based on RTL-SDR, use the aft rail to mount the antenna. Less signal loss, less cable required, less hassle and unnecessary to climb the mast - no bruises.