. . . battery went dead . . . there is no charge when the engine is running . . . have the original voltage regulator . . . What should I check first and how?

Drive down to the boat.  Take the battery out of your car/truck.  Put it in the boat.  Go Sailing.  Come back to the dock at the last possible moment.  Give the boat's battery and charging system some
more thought tomorrow or next week,

    Check the connections.  It has been my experience that small amounts of corrosion play a major part in electrical malfunctions.  Particularly on the ground side.  I would remove and clean all of the connections on the charging side of the wiring, and see if that fixes your problem.

   The generator needs 12 volts to power the field coil in order for the armature to generate power.  When the field coil is powered and the brushes are working, the generator output should be around 13 volts out of the regulator.  Sometimes, you need to reset the points on those old regulators. Remove the cover, and with the engine running at more than idle, close the points.  They should stay closed and start charging showing the 13 volts output.

    A secondary issue is, why did the battery go dead when it wasn't being used (?).  Sounds like you have a short or left something on.  It may be that the battery has an internal problem and is shot.  If it takes a charge and cranks the engine over, it's probably okay and you'll need to look farther down line.  My first step in chasing elusive lost electrons, is to disconnect everything, put my amp meter in the power side of the circuit.  Then as I reconnect each circuit, I see if there is a draw with the switch off, and if
the draw is about right with the switch on.  Tedious for sure, but in the long run conclusive.