We've been using our semi's (5mm + rash vest) for the last couple of weeks in Gildy, temps around 17ish in the first 12 metres and it drops to maybe 15 or so deeper, we've taken our dry suit feed off but others just wrap them around the first stage (just in case they don't like it cold
and want to wear the drysuit for the second dive) I'd give it a go and see how you get on, maybe take the drysuit just in case?