There's nothing special about "offshore" or even marine gear, other than the price AFAIK
. The same type of extremely waterproof/windproof clothing is also marketed to motorcyclists, and backpackers at much lower prices.
For outer layers I use Helley Hansen VOSS jackets and pants which are extremely durable, 100% waterproof/windproof, and typically cost about $30 each! They're made from welded polyurethane, which is extremely tough, and stretchy.
For middle (insulating) layers, I use army surplus felted wool clothing, which retains insulating ability if soaked, and costs $5-10 per garment.
For inner layers (against skin) I use IBEX super soft merino wool, which is expensive but lasts forever and works great for non-sailing clothes also. Cheap polypropylene army surplus "extreme cold underwear" could be used as well.