If you want to avoid petroleum based neoprene, Patagonia makes wetsuits intended for surfing that if I recall correctly are branded Yulex. Henderson makes "greenprene" which I think is also sans petroleum. Nothing that I know of made in a "more" environmentally friendly process will be below market rates for other wetsuits, they are usually more expensive.
Sharkskin products are textiles, and I believe they are made of synthetic fibers, also manufactured using petroleum products (just like most clothing we wear now). They advertise being environmentally friendly.
The option with the least environmental impact is to buy or find something locally that has already been made, and would otherwise be discarded.
I have heard that some swimmers will use grease on the skin to provide insulation, though vaseline is of course a petroleum product. This site (1) recommends against grease coating, then suggests soaking a wool vest in freon gas before coating it with grease. That is an interesting idea.
Human fat is insulative, so a sustained caloric surplus could result in some resistance to cool water. Reference (1) prefers this method to the greased pig look. The environmental impacts of this method are difficult to calculate, but it seems to work for the seals.
Another option is to pray to Wim Hof and try not to think about being cold. That is probably the most environmentally conscious approach.
All manufacturing and human endeavors have environmental impact. Reduce, reuse, recycle, and find a wetsuit that keeps you warm and safe.
(1) (
https://www.soloswims.com/grease-baddeley.htm)