Checked out boat last night & battery was dead. Battery was new this spring last used a month ago no problem then. Put charger on & in an hour battery was at 11.6 volts (8 amp charger). I need to know how
to check for current draw on accessories. Two switches on dash,power trim & tilt, VHF, fishfinder.

Lowrance's external antenna draws some current if it is left hooked up, they suggest you hook them to a switch to disconnect them.

Unlikely the antenna - it's draw is very small. With a mid-sized battery, it will take 70-80 days to drain it. Good to put it on a switch, still. I have mine rigged to my master accessory switch. Only the TnT and Starter work without the switch being flipped.

Go to Radio Shack and buy a multimeter. Buy very cheap, you don't need fancy bells and whistles. Ask the guy there to show you how to set it to read amps. Note that it will have several ranges, from multiple amps to fractions of milliamps. ALWAYS start measuring with the high range at each measuring point, and work down in to the lower ranges to verify there is either no or little current flowing. If you start in one of the very low ranges, and have lots of current, it could damage the meter.
Start by disconnecting one battery terminal, and put one probe on the cable end, and one on the battery terminal. Turn down your range until you get a reading, or it always stays at zero. Note in the smallest ranges, you may get a really tiny reading (.001 milliamps or such) - don't worry about those - that is an artifact of the meter, I did advise you to buy cheap!). DON'T try to see what the reading will be if you hit the starter - there will be so much current, your meter will go up in a cloud of smoke! (OK, maybe a little dramatic - but don't do it...).
Connect the battery again, and now do the same test for each accessory - unplug one power wire, and put the wire on one probe, and the other probe on the power connection, and note that power consumption. Or, you can leave the initial test setup in place (meter between main battery cable and battery terminal) and unplug devices to see how the current draw changes - that may entail some running back and forth in your boat. Whichever way is easier is valid.

To figure out how many hours your battery will power a particular device, or combination of devices - divide the Reserve Capacity of the battery by your reading (or sum of all the readings, if you have multiple devices). So, if your antenna draws 100 milliamps, and your battery has 180 amps RC, that would be 180/.1 or 1800 hours or 75 days to kill the battery.

BTW - your battery will show a fast increase in voltage with even a short bump on the charger. But, it will have no life - it will die totally again very, very quickly. It's got to get a complete long charge cycle before it can be expected to have much endurance. But, that subject is a whole different discussion.....

I had a similar problem, but in my case, I forgot to turn my radio off. The memory drained the battery. I installed a KILL switch on the battery. Now, after I park the boat, I turn the battery off. Never had a problem since.

I have a master switch for my electronics and I installed a battery cutoff switch to keep my outboard from drawing the start battery down when it is in my garage for a period of time. It has worked great so far.

Some EFI motors, especially 4 strokes will drain your battery over time. Like everyone has said. A master shut off switch is a necessity. It is more inportant than your locator. No power-no fishing!

Used boat today & my sonar went down. Its a Lowrance 135, the screen turned brownish like the 125 there was also a dark square in the upper left corner of the screen just before it shut down. I think the sonar was what was draining my battery.

It sounds like your battery - and the sonar went down because of low voltage from the battery, not vice versa. If there was anything wrong enough with your sonar to discharge a full battery in one fishing trip, you'd see smoke coming from it, feel the heat, maybe melt the plastic case, and the sonar won't work at all anymore......

The boat sat for a month or more when the battery was discharged. When I went out yesterday it was with a
new fully charged battery.