Separating coil/Distributor power from everything else.

Carburetted 5.7 I/O, formerly Mercruiser, I bought the boat with a gaffed engine electrical harness. I pieced it back together and got it running well, but I've been nagged by a problem with the engine losing power and dying. Been towed back 3 times. The last time was on a troubleshooting run with a tech. Boat was running great but as soon as I used the trim tabs the engine lost power. After that it wouldn't run well at any speed, died, and wouldn't restart and we got the tow of shame back to the marina. After it died, the tech checked and said the coil was hot. I pointed out that the same wire going to the ballast resistor also fed the Mallory breakerless ignition, as well as being tagged off of to run an aftermarket electric fuel pump and other stuff (apparently the trim tabs). All told there are about 4 different wires all tagged onto the hot going to the ballast resistor. The tech said that there is just too much load on the single key-on power wire and the coil is running on low voltage, which is heating up the coil and causing it to shut down. This was proven a previous time it died, when I put a timing light on the coil wire and was getting maybe one spark per rev while trying to start.

So my question is, where to put all the other loads to isolate the power to coil and ignition? The wiring diagrams I've seen show a purple wire running to the coil and ignition. Since the engine is a PMC replacement with aftermarket ignition, the wiring has been "adapted", and some wires were combined. The tech suggested I take a tap directly from the battery to a relay for the fuel pump, etc. and actuate it using key-on power. But is there a more OE way the ancillaries should be powered?

Depends upon the 'ancillaries'...as a rule, keep the 'engine stuff' isolated from the 'hull stuff'. as far as the 'engine stuff' - your tech's suggestion is viable.

On the fuel pump, make sure it has one of the recognized ways to satisfy the shutdown requirements - typically using an oil pressure switch.

Also, make sure the ignition switch is in good shape as well as the main harness connector...

Ayuh,..... Other than the trim tabs, which should be wired to the battery, the other things are normally wired to the ignition circuit,....

To test yer tech's theory, bring along a long enough wire with spring clips on it's ends,....
Use that wire to run from the battery's (+) post, to the coil's tiny (+) post,....
If ya don't have spark then, something in the ignition system is failin',....

I'd suspect a bad ground somewhere, 'n would remove, 'n clean both battery cables,.. Both ends,....