I used to be able to rattle this off from memory... Best I recall Yamaha bought Skeeter in 98. They used their engineer team to redesign the transom and stringer system. In 98 some models have a "c" designation to indicate composite. An example would be a " '98 Zx185 c". This boat would be composite while one without the c would not be.
I believe by 99 or 2000 they were all composite. A common misperception is that composite meant wood free. This isn't the case. It just applied to the stringer and transom but not the dividers between compartments.