I’m in an academic career related to the sea/ocean but I actually spend all my time in front of a PC, so don’t live anywhere near the sea. Yes there is lots of opportunity in both instrumentation and emerging robotics / automated observation platforms and infrastructure, for science, Earth observation, Government agencies involved in things like fisheries and in the private sector in surveying, oil & gas / resources, renewables, ports operations etc. For the science side, instrumentation, electronics in harsh environments, battery reliability and sat-comms are primary things to know about.
IMarEST is the established institute who can give an idea of the breadth of opportunities and help with what kind of training / qualifications you might need, e.g. CPD.
If you actually want to be at sea or do something adventurous you could look for opportunities to join research vessels or be on engineering teams that deploy (usually short stints at sea, but can be longer cruises). National Oceanography Centre engineering mostly operate out of Southampton but also have a site in Liverpool that does coastal work, Plymouth Marine Lab, Scottish Association for Marine Science (Oban), and British Antarctic Survey have a polar oceans team (they tend to look for 6 month or 18 month seasonal station staff around this time of year to start training in September). Bangor University (N Wales) and Uni of East Anglia (Norwich) also run research vessels / UAV but I suspect being quite small groups they are more likely to want people already trained specifically in oceanic engineering; whereas places like BAS will train people up for what they want, it is more about your personality.