I've used a BodyGlove hooded vest ~5mm under a 5mm S-Tek for warm water to
about 20?C.

Initially bought the vest as I got really cold with water flushing through
my suit when snorkelling / freediving. Later realised that that wasn't going
to solve the problem, but it is nice for deeper colder scuba dives as it
stops that little creep of cold water in through the neck and down my back.

The hood is a little tight so I may lop that off at my jawline, and just
keep the long neck.

S.C.