I contact Larson a couple of years ago to see about a manual for my '97 boat, and they were very kind and looked to see if there were any old copies available. No luck, though; they said the company had been sold a couple times since then, and no old copies of manuals (if there were any left over) were kept. I would guess the same would be true for your '89 model, but you could always email them and ask.
Boat manuals tend to be pretty worthless anyway; they contain mostly safety warnings and instruction on boating etiquette and the like. Maybe a bit of model-specific info on what the dashboard switches do.
Is there any specific info you want, or were you just wanting to see what is in the manual?