1. how "water resistant" are alternators? i've seen pictures of isuzus playing in water. do they escape unharmed or are they sure to have electrical problems?

Most alternators are not waterproof, unless you get into the marine realm. Also, most alternators can handle some water with little problems, but...most water off-road is full of sand and silt. Those kill alternators very quickly.

2. are the isuzu 4 cylinder alternators located relatively higher than the 6 cylinders? the reason i ask is that most of the isuzus that i've seen with snorkels have been on gen1 troopers. that being said, i know that snorkels are available for the gen2s.

Nope, they are in about the same places. As for snorkles, it is probably more related to those people who want to cut holes in their fenders.

3. is a snorkel overkill? i mean will the isuzu get into trouble before water even has a chance to reach the air intake given the alternator's relatively low position?

Yes, no. Depends on the other waterproofing that has been done.

4. why is it necessary for the volts to decrease as the engine is under load and why does it become unnecessary when the lights are on? thanks, taro

Amperage is RPM dependent. Voltage is regulated by the regulator in relation to the electrical load. RPMs keep voltage up, therefore an engine under a load usually drops in RPMs and therefore voltage/amperage drop to a certian degree.

Good luck,
Michael