Re: '88 Mercruiser 3.0 Seized while underway
You can take off the top cover and take a look inside at the top gears if you want. You need am OEM manual if you want to go further.
Just be realistic here...you aren't going to be able to fix this yourself. Best to let someone with all the proper tools and knowhow to take it from here or find a used or rebuilt unit.
When you do get your drive fixed, learn how to properly fill these things. They are different than all the other drives.