Is there a tension release bolt somewhere or do you just pull the new belt up over the plastic pulley.

I think I see how it works. The pulley moves up and down when you loosen the pulley bolt. Is this correct?

Yep, The large nut is the lock nut. The inner stud is what moves the adjustment up and down.

Loosen the lock nut, turn small bolt until belt tension is correct. While holding the small bolt with a 5/16" wrench or socket, tighten the lock nut with a 5/8" wrench.

Thanks for the verification. Any recommendation on where to get belts. They cost a fortune at the boat shop.

Any auto store. Take yours with you to match.