I've just done some re wiring on a 20 year old motor.
It wasn't in too bad shape, in fact it was overall in very good shape. The reason I did it was I was getting no charge on the battery, turns out I needed a new rectifier. Whilst I was waiting for the part I decided to check all the wiring. So far I have only done the engine, control loom and kill cord.
I found that the kill cord did'nt work at all as the wires had pulled off the switch. I disasembled the switch and rebuilt it that works okay.
The main problem I came across was where the control wiring entered the engine along with the wires from the battery. This had been crimped overtight and in fact had snapped one wire and another one was about to go.
This has all been repaired and everything seems okay now. Next job is to do the wiring on the cockpit area.
It would seem that the first thing you should do is visually all your wiring looking for anything obvious that might cause a short. In particular check between the engine and your control/cockpit area. The thing with boats is that they are not built to a precise procedure like cars etc, but assembled at the dealers in a myriad of combinations.
I suspect the fuse is actually blowing when you start or shortly after starting the engine.
Take a look at this site. Tell them your problems and see what they come up with.
I'm not too sure how to post a link in this forum but you can copy and paste the above in to your address bar and take it from there.
Also if you need to buy some spare parts can I suggest that you get them form the US of A, its a lot cheaper than here. Do a quick search on E Bay for the part you need and see what prices come up.
For instance I had to buy a Rectifier. UK cost over £60 new. One second hand one on E Bay for £35. In the US second hand cost me £11.50. No import tax but I'm still waiting for the postage bill.