An industrial control system (ICS) / SCADA security specialist is a great option. This is something I was involved in personally. There are fewer jobs than a none specialist but they are well paid and well respected. There is still a shortage of people with ICS and security experience. Still, within ICS security, you have to choose where you might specialise. So you might be an ICS penetration tester, engineer, architect or analysts etc.
Look into these standards and resources to help you
* Society of Automation (ISA) 95/99, - builds on the guidance in these ISO/IEC 27001 standards.
* NIST 800-82 guide to ICS security, 800-53 recommended controls - like iso 27001/2
* ENISA (European Network and Information Security Agency),
* UK National Centre for the Protection of National Infrastructure (CPNI) https://www.cpni.gov.uk/
* North American Electric Reliability Corporation (NERC) Critical Infrastructure Protection (CIP) Standards (003 – 009)
* ICS-CERT
https://www.amazon.com/Industrial-Network-Security-Second-Edition/dp/0124201148/ref=sr_1_3?ie=UTF8&qid=1414970315&sr=8-3&keywords=industrial+network+security