I have marine foul weather gear for the warmer seasons I usually sail in. For really cold sails I have used ski gear which worked just fine - Not optimal but certainly good enough that it was nowhere near worth it to spend the hundreds of dollars it would cost to get marine foul weather gear for that range of temparatures. As marty said above, the snowboard pants have the right reinforcements - Mine are shells to begin with and with an assortment of long underwear and fleece pants to go underneath I am all set.
Two things to do though
1. Buy a roll of reflective tape - any bike store will have it
2. Get real boat boots - sneaker or sandal types wont cut it - you want real boots that are tall enough that when you slip the ski pants over them there is no way water can get inside of them