Two of my gauges are reading incorrectly:

1. Voltmeter (has had this problem since I bought the boat 2 yrs old): When switching on the Capt Call, the reading drops from 13.2V to around 11.5 - 12.0V (the actual reading off of the battery is 14.2V, regardless of whether the Capt Call is on or off).

2. Fuel gauge (new problem): Reading drops about 1/4 tank when I switch on the Capt Call.

Any advice?

Sounds as though your guages are fine. Problem is probably a bad connection somewhere ( either + or ground) . My buddys boat did the same thing last year. Found numerous problems .
Found a loss of .1 volt or .3 volt even .4 volt. By the time it went from the battery to his guage he had 11.5 volts yet the battery was reading 13.6v.
The same problem is probably effecting your fuel guage also. The voltage drop up front is causing that to drop too.

I had a similar groung problem once that caused ALL my guages to change. In that case I ran an extra ground from the battery forward to the instrument cluster...that cleared up the problem
Good luck, if you don't know what you're doing , electrical problems can be a nightmare

My buddy was also having starting problems. Sounded like a bad battery. VM on battery showed 12 v cranking but on the starter a little less than 10V. That turned out to be corrosion under the crimped on terminal at on end .

The easiest way to find out what is wrong is to hook up a set of jumper cables to the batterie that will reach the dash. Then use a DC voltmeter between the jumper cable hot and the hot wire on the key switch with everything turned on like you said and read the voltage on the meter. If the wire is big enough or all the connections are good then you wont see any or under 1 volt reading. Next hook the meter to the negative jumper cable and touch the other meter lead to the ground block for the dash - again if everthing is OK you will see less than 1 volt on the meter.
( a digital volt meter works best for this job. ) If both sides show more than 1 volt than check all connections between the batteries and the dash. Also the wires may be too small because the wires were sized for the original dash and more stuff was added.

Actually, all of the stuff that requires power was original equipment on the boat. Even so, you may be right in that Baja may not take these options (Capt Call and trim tabs) into account when sizing the wiring to the dash.

Anyway, I will try your suggestion. Just hook + or - from the multimeter to the + on the jumper cable and then hook the other lead from the multimeter to the power wire on the gauges?

What you are doing is making a second power path for the guages or the dash as a complete unit. If you have the proper size wire ( either + or - ) and all the connections are good you will have almost no volt reading on the volt meter. the wire that is too small or has bad connections will show a voltage reading on the meter of more than 1 volt DC. You go from + on the batterie to the positive on the switch or the meter and if it reads more than 1 volt than the positive wire or the connections are bad. Do the same test on the - side.