50ºF is a tough temp. Just a bit too cool for polypro and a bit too warm for fleece. Maybe 2 layers of polypro--one lightweight, one heavyweight, or lightweight polypro + very thin fleece?
I like just polypro under my drysuit in the high 50's-low 60's; 30's-40's are solid fleece temps, depending on exertion/expected immersion.
....and a neoprene beanie or hood under the passenger's helmet will also help regulate temps.