Good luck -- most manufacturers don't publish them even if they had one. Their instrument panels are generally produced by an outside firm so only they would have a diagram for assembly purposes. Finding out who builds the panels is not worth the effort. There are diagrams in the "stickies" in the ELECTRICAL forum. Boat wiring is very generic so the diagrams should be all you need.
Yup, you are on your own here, boat aren't like cars to where you can get all that stuff just because they are bult in such small numbers.
I build pontoons now and woudl be suprised if they could find a diagram for a current boat, the wire harnesses are farmed out and come in with a model number on them and they go on that boat, noone ever sees a diagram.
Label wires before you remove them so you know where they go back or if you are replacing make your own diagram of what is under the dash but you aren't going to find one anywhere.
I suggest that you first check, clean and/or repair all questionable connections. Be especially wary of common connections, (black wires that connect together and ultimately run to the negative terminal of the battery). Then check out everything carefully and determine things that do not work properly. Deal with one malfunction and a time. If your electrical skills are minimal, then seek out a competent boat technician and/or post your problems on the forum.
The electrical system on a pontoon is actually quite simple.