Maybe the fuseable link at the starter if it has it like merc puts on it is a white block on the large. Battery post on the starter it’s to protect the ecm and accessories on the engine. Pretty much if it’s bad the only thing that works is cranking over.
Generally if the boat can run 70 plus you should have hydraulic steering main reason at speeds 70 and above you can get prop blow out where the prop will make a void or pocket and put the drive in a turn you can’t hold back
When you get the seat out you will probably have to remove the rams and take them apart sound like they went to far down and bottomed out you have to hold the large gear and put a screw drive in the top hole of the ram and break it loose then put it all back together and it should work they make...
What kind of distributor is it m s d is the distributor the same one or has it been replaced if it’s been replaced did the problem start if it’s m s d They make a filter for it
Because the real propeller blow out can happen at speeds of 70 mph plus blow out is when the prop washes away the water causing a void or pocket and will throw the drive into full lock turn faster then you can react. Hydronic steering will help keep drive on track.
Could be the fuse able link on the starter as said above or under the plastic cover on the engine on port side by e c u there is a black cap inside of that there is fuses