such as the Fluke 325 or Extech DC400 - there are many on the market. These clamp around any wire you can reach without disconnecting it, and read DC amps from around 10 mA up to 200 amps or more. Make sure you get one that goes down to 10 mA DC, very useful for tracking down spurious drains, etc.

Your glow plug issue is a case in point: by clamping the meter around each wire in turn, you will see if they are all drawing about the same current. And get it done in 5 minutes. Without swearing (much). You could also clamp it around the starter cable to see how much amperage the starter is using, might be going bad. These things have uses right behind a DVM, and if you get one like the Fluke 325 it is also a very good DVM, ohmmeter, frequency meter, temp meter, capacitance meter, etc. - so one handy tool does it all. I would not be on a boat without it.