Reddit reviews uxcell 85C1-A Analog Current Panel Meter DC 5A Ammeter Ampere Gauge Tester

Most multimeters have a current setting, the part that says "mA" or just "A". Pretty sure even the ancient analog boxy-looking ones have it. When measuring current you have to be in series with the flow of electricity (as opposed to in parallel for voltage), which is why you need to break the wire.

The less intrusive way would be to pick up something like this, and solder jacks to the input and output side which is basically what Truetone have done.

Granted, you're obviously paying extra for build quality and whatnot , but I think $20 or so is the most I'd pay for something like this.

Since you can't easily measure current without being 'inline' with the circuit (hard to do with most multi-meters since they typically won't pass a very usable level of amperage), I would recommend testing the voltage on the battery when it is disconnected, then measure again when connected to the boat, if the voltage drops a few decimals then there is current draw.

They do make inductive type ammeter that allow clamping over a wire, this could be helpful if the current draw is enough to register, here are a couple for cheap:
http://www.amazon.com/0-10A-Rectangle-Analog-Panel-Ammeter/dp/B0050MQK16/ref=sr_1_5?s=industrial&ie=UTF8&qid=1404957457&sr=1-5&keywords=ammeter

http://www.amazon.com/85C1-A-Analog-Current-Panel-Ammeter/dp/B0052C9MMI/ref=sr_1_9?s=industrial&ie=UTF8&qid=1404957457&sr=1-9&keywords=ammeter