Electricians...I have a mystery

It is almost impossible to get a hold of an electrician in my area. They don't return calls and so I've got a "contractor" buddy helping as well as he can. Not a professional but he gets by.

This is a problem we are chasing and if anyone can help to let me know if we are on the right track that would be cool.

-The main breaker was tripping inside the house box when we had what we thought were too many things running.

-We chased the problem through all the circuits and discovered that everything worked when the hot water switch was turned OFF.

-Buddy came out and checked the inside box and nothing was odd there.

-Checked the outside box and the main switch out there was toasted. Half melted actually. So, we switched that out and everything seemed ok.

-About a week later we used a lot of hot water for laundry, dishes, etc. We also had the dryer and AC on and suddenly....the whole house went out. We checked the Main INSIDE the house and it was fine. The main OUTSIDE was HOT and arced when we flipped it back on. 45 minutes went by and it tripped AGAIN.

-Using the trick from a week earlier we shut off the hot water switch in the house. Nothing tripped again and everything else worked.

-Our buddy came back and checked. With everything on we can think of that draws a lot of electricity we reach about 45 amps and the outside main trips. It is a 100 amp box.

-We did also discover that our water heater was shot. Not sure if this is related. The water heater alone draws about 16 amps.

-We just got a new water heater and have not installed it yet.

-Our buddy is thinking we need a new outside box all together which he is able to do for us.

I know it's a long story and there might be nobody that can help or cares to. If there is anyone who likes an electrical mystery feel free to offer your two cents worth.

We are already moving forward with changing the outside box (when the rain decides to stop) but I want to know if we should be prepared to have further issues once the job is done.