I have worn an old fashioned yellow rain suit over everything else I had on. It kept me dry. Think Gloucester fisherman......
The trouble with the traditional yellow suits is that they are coated and not breathable. If you are reasonably active for a bit, working on the deck, for example, the perspiration you generate will end up soaking the inner layers of clothing, which will guarantee that you get cold.
The breathable stuff is expensive (perhaps $700 for a full suit) but worth every penny IMHO.