If you’re fed up with getting a cold and wet bum and thighs but don’t want to go down the mudguard route, these Paclite shorts from Gore Bike Wear could be an option.
They're super-lightweight, breathable and waterproof, effectively keeping the water from your nether regions as you ride. The drawcord legs can be tightened to keep water from going up your leg and a drawcord waist is easily adjusted. We’d prefer to have had an elasticated drawcord so you could leave it adjusted to your size, but it worked fine.
The light material doesn’t add up to a heavy short so you barely know you're wearing them when on the bike, other than the rhythmic swish noise, and it also means they have the ability to just disappear into a hydration pack, ready to pop out when required.