93 omc 5.0 cobra throws alt belts

already replaced alternator. first time was with the incorrect alt and the fan was backwards causing the alt to get very hot very quick
i found the right one and installed it. customer drove the boat around the lake several times with no issues and then i got the call today that the belt burned up and broke again

i dont see any missing brackets.
the pulleys are all alligned well.
the engine water pump is fine with no wobble or leakage
no oil is leaking on it.
the customer claims its been doing this since he bought the boat and he had another guy looking into it and could never figure it out.

now im stuck with it
help please.
what am i missing about this issue.
is there supposed to be a bracket from the back of the alternator to the intake, or something, to keep it from flexing and throwing the belt?
is there a special belt tightening procedure to make it just tight enough to not squeal while preventing it from flexing the brackets and throwing the belt?

Ayuh,.... Rusty pulleys,..??

pulleys are clean. i filed and sanded them smooth to make sure.
i think whats going on is that when i pull the belt tight, its causing the alternator to cock to one side and throwing the pulley out of align.

i just have no idea what to do to fix it.