First I want to sincerely thank all of you for your recent and helpful responses to my questions. The boat is running well now. But! I found on our first ride the batteries never showed the charging voltage of 13.5- 14.5v. Both batteries are new and charged. I did change the 25 year old alternator last year for a chinese "scorpoin" model 😶....... maybe a bad move. Anyway, so it did work on the 3 rides last year after installation. Not this year tho. Except I'm also wondering, since it's new, if maybe a wire or fuse is bad ? So I checked voltage at the alternator on the pole with the orange wire. With engine off it measured the same voltage as on the batteries. With engine on this pole went to 15.8v. I shutoff the engine, tried again and it showed 12.7 as the batteries showed.....do I spend $500 on a new mercury alternator or is there a wire/fuse issue here? Thanks

Or possibly the wires are not in the correct positions ?

Looks like the red (sense) and purple (excite) wires are correct as is the orange (output) wire. On mine, the ground wire (black) is connected to the terminal labeled as "E" just to the left of the red wire in the pic. I don't see a black wire in your pic.

What's the wire with all the tape on it connected to?

ok

Mine also has a black to Earth (E)

If I'm reading correctly, with motor running it measures 13.5 to 14.5V at the ALT (orange wire) but does not read same at helm gauge.
If correct it's most likely the gauge

15.8v is too high and will fry your batteries