All these unit are bus or truck units and expect to be operated at 14+ volts.
Bosch glow plugs are rated for 7.5V but will operate at lower voltage , as little as 6.
The Espar folks did not expect multiple (thermostat) starts , just light it and drive.
There is a large coil of nicrome wire on top of the unit that reduces the voltage to the glow plug.
After you wire the unit in the boat system with #6 or #8 wire , measure the voltage at the glow plug during a start sequence.
Cut usually 2 turns off the wire coil if the voltage is below 6.
You want to see about 7+ v during a battery start.
A fuel tank with 50% to 100% kerosene as fuel is a huge help if its a liveaboard and will see extended winter starts.
Truck heaters do work in boats , but have to be set up for the service.
If slow starting has been chronic it is likely that the nozzel and set of washers attached are gunked up.
Replace them with the annual overhaul.
Some folks will tale off the duct work and block the output.
This forces an over temp, no big deal its got protection , and after 3 or 4 overheats much of the carbon is burned off.
Just watch the chunks fly out of the exhaust.
In NY Bosch would charge $75 for a tech to come and simply over temp the unit as a "cure".
The real cure was much heavier wiring as the unit will draw 40A or so during starting .
Good luck, and remember slowing the fuel delivery , "full or half "on the operating plate is a better temp control than off / on.