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Don't worry about the 'filled with water' sensation - they are filled with thin oil which circulates passively and cools the windings inside.

I've had coils go bad on me in a number of ways. Sometimes old coils gradually start making weaker and weaker sparks, even when the points-n-condensor is in tip top shape. This generally manifests itself with a sort of growing weakness at full throttle (I presume the high cylinder pressures are harder to push a spark through), but is evident even at idle by observing the spark. Fat is good, thin is bad. Sometimes the coil developes a tendency to stop working when it gets hot, so the car starts and runs fine, just far enough to get you too far away from the house to be handy to walk back, and quits until it cools down. And finally, old coils sometimes develop cracks in the top that insist on shorting the spak when the humidity level goes over desert-like conditions.