Try this, hopefully I can 'splain it sufficiently. You'll probably need to remove the battery from the boat:
1) Fill all cells with water if they're low. Distilled is best.
2) FULLY charge the battery. (Don't set it on the concrete floor of your garage when you charge it).
3) Remove the caps. Using a voltmeter set on "DC" put the negative lead on the negative battery post. Put the positive lead into the first cell ( the one closest to the negative post) lightly touching the plates. Record the voltage.
4) Now put the negative lead into the first cell, the positive lead into the next cell. Record that voltage.
5) Continue until you're on the last cell (negative lead) and the positive lead is on the positive terminal.
Now, the add the FIRST reading and the LAST reading (one will be high, the other low). Divide by two. That number and all the other numbers from the cell to cell tests should be within .2 volts of each other. If you have a shorted cell it'll read really low, like your cells will all be 2.2 volts but cell number three will be 1.4, or some such.
If the numbers add up and there's no significant variation the battery's OK and doesn't need to be replaced.
This is a much more dependable test than the specific gravity test, which doesn't tell you anything other than the battery's charged. A cell which has shorted plates will have a lower voltage than the others; the battery should be replaced.
Or...
Say you have a shorted cell, you can try this trick which sometimes works: Dump all the old cells out somewhere (I like my neighbor's yard, yuk yuk). CAREFULLY refill the cells several times with clean water and dump it. After you've gotten all the acid out put a couple of spoons of baking soda in each cell, then refill with water. After it quits foaming dump the cells again, rinse, repeat.
You may need to do the baking soda thing a couple of times - sometimes that'll clean the sediment out and "unshort" a bad cell. Now go to the local auto parts store and buy a box of battery acid (its liquid but they sell it in a box, like fine wine). Turn the battery upside down and let it dry overnight, refill with battery acid, recharge, then repeat the cell to cell test.
This only works about half the time but baking soda and battery acid are cheap, it won't cost you nearly as much to try to "rejuvinate" a dead trolling motor battery as it'll cost you to buy a new one!
Let me know if this works for you.