My last ride of last season was similar, about 50/70 with just a heater top... And my two friends were just trunks. A little shock when you get in but quickly forget about it when you're riding. I figure it's incentive not to fall.
Should be out riding by the end of the month when the river gets up to 50*, but this year I have a full 4/3 suit so should be toasty warm!