Height above sea level is important because the combination of your antenna height and your co-respondent’s antenna height determines range. Your horizon distance plus his horizon distance are what’s important – they need to touch to communicate. If you have a very short horizon distance you'll be relying on his horizon distance to reach out to you, so you'll only communicate with very high antennas such as on large ships or at coastguard stations.
Mount your antenna as high as practical – mast top on a yacht or on a pole for a RIB or other power boat.
Your output power is restricted by law, so make sure you don't waste it. If you use undersized cable, have corroded cable, have badly made connections and use a cheaply constructed antenna you'll rapidly lose your signal power.
Use RG8X coaxial cable for runs longer than 6m and RG8U/RG213 or better for runs over 25m. RG58 cable should only be used for runs up to 6m. Cable must be tinned copper for marine use.
Keep the antenna vertical, you want the signal to point at the horizon, not sea or sky. Using a long fibreglass fishing rod antenna leaning backwards might look sexy but it's useless for communicating.
Use a whip antenna – its fat radiation pattern will ensure you always have some of the signal pointing at the horizon even when the boat is rolling about. A 1m whip antenna on a 1.5m pole is much better than a 2.5m fishing rod antenna.
Keep the antenna away from other antennae (about 0.5m minimum) and away from vertical bits of metal.
Make sure that your masthead LED navigation lights don’t interfere with reception – some do, some don’t, so check. This is particularly important for AIS because some of your targets might disappear but not others, lulling you into a false sense of security.
Antenna quality counts - the gauge of the loading coil and how well it's supported effect transmission. Don’t scrimp on antenna quality if you want to maximise the performance of your radio – a penny spent on the antenna is worth a pound spent on the radio. Get a Metz or an AlphaOne – they are superb quality and have lifetime warrantees on the coil but cost no more than many ordinary antennas.