I have mine in a box with all sorts of gizmos!
It has a small LeadAcid battery incase connection to main battery disconnected.
A large diode to stop the engine start pulling on this battery.
A surge protector as I believe the lighting coil on my OUtboard once spiked (as they do) and popped something on the circuit board (easy replacment luckily!) - http://www.sourcingmap.com/12v-power-suppl...or-p-98360.html
You communicate to the via a phone call and \ or text message.
You can set it to alert you by sms if the unit moves 200yds from where you left it - handy, but remember to switch that feature off before you take the boat out, or you end up getting alerted you are out and about (this is switched off and on by txt message command).
You can even wire in to the engine and send a command to open a relay to stop a fuel pump of similar and remotely disable the engine - i think that is a bit much of a risk to go wrong on a boat!
When you txt it - it advises you of location coordinates, speed, direction ect and you get a link to a Google map showing the position!
You can also ask it about battery voltage, gps and cellular signal strength. Ask me tonight and I'll show you the communications on my phone.
As it uses any pay as you go sim, it costs up to 10p per text - depending on tariff and you must use it every 180 days or you can loose any credit on the account and \or sim is disabled. Ensure you get the right PAYG package - not a top up monthly one!
Rob