I understand that there are two different ammeters that can be used:

1. Ammeter which is a direct unit. This type goes inline between the battery and the alternator. The problem here is that the line must run all the way up from the alternator to the gauge dash board and back to the battery. This can be a loop from 30 to 60ft long depending on the boat.

2. Ammeter with a sensing unit. The sensing unit (induction type) goes in line from the existing wire from the ammeter to the battery. A separate signal wire is then routed from the sensing unit to the gauge dash board. This is preferable.

Which do most boats use?

Ammeters in a boat are, in my opinion, dangerous.

I prefer Voltmeters.

Just my 2 cents

An ammeter with a shunt resistor. The resistor and the meter are wired in parallel. Only a very small percent of the current goes through the meter, most goes through the shunt resistor. Most high amperage units or more expensive units are this type. No need to run a huge wire to the dash.
Gary

What Gary said, voltmeter is usually adequate though.

I agree with formula31 , if it's charging it will show in the voltage. If the voltage is dropping , it's either not charging or not charging enough. In either case attention is required.
If it's over charging , say over 13.4v for an extended period , you will see that also.

I actually like having ammeters on my cruiser. It's easy to tell if I left something on, lights, water, etc. Also helped me diagnose a diode that was going bad in the alternator. But not really necessary if you've got a volt meter.
Gary

Use a volt meter. Simpler wiring,less danger of electrical system damage if the meter were to short internally(yes this does happen).