Got a 2000 Ranger 619 Fisherman, all of a sudden my cranking battery has started going dead in like a week.  The battery was 3 years old so I replaced it but now my new battery has gone dead and I charged it when I got off the lake Monday and today it's dead.  I'm talking dead dead, won't even toot the horn dead.  I usually forget to turn the breakers off cause I'm a dumb ass but it has not been a problem for the three years I have owned the boat.  I have not messed with any electronics on the boat this year and it's been out several times.  In fact I went out for a 4 day trip the week before it happened and didn't have a single problem.  Got home Sunday charged it, put it in storage and it was dead by Friday. The battery won't even register on the on board charger unless I hook up a regular charger to it for a minute.

How should I go about trouble shooting this?

Diagnosing battery drains can be sucky. First easy step is to make sure everything is turning off or even turned off such as lights and what not. If nothing jumps out time to hook up an amp meter in series with the battery (post to wires) You should see an amp draw. If that doesn't eliminate boat vs motor (don't kniw which wires are what) disconnect the outboard harness if that doesn't stop the draw then it's somewhere on the boat harness. Start pulling fuses one by one until you find the one that stops the amp draw. Now you know which circuit is the problem so you can move your meter and test  switches and gizmos on just that circuit.

Multi meter set to DC amps inline with the battery cable. That will show any current draw with key off. Start disconnecting accessories and electrical components until the meter reads "0". Could be the key switch itself.


ETA: damn, beat by 2 tenths of a second

I figured it out.  Went out to see what the charger was reading and my dash lights were just a glowin.  Dad just started launching and landing the boat as his ballerina skills are even worse than mine and I was worried about dunking him in the lake again.  Anyways he was just turning the motor off but not turning the key all the way to the off position.  Maybe not 100% his fault as my ignition had kinda got loose the last couple times we were out so while I was up there I tightened it up.  Once I get back from tomorrows outing and get a good charge on it I will do a draw test for shits and giggles just to make sure I've got the problem solved.

Thank you for the responses though.