Signals Intel O's manage intelligence marines who conduct signals intel in various ways. The officer doesn't usually do the cool stuff, they normally support the enlisted guys by doing their paperwork, making sure their family/pay/medical/logistical issues are met and that they have what they need in the field to operate. Their marines have eavesdropping equiptment and will go in the field to forward operating areas in humvees and LAVs and pick up comm traffic for analysis.
They go for a crash course at the communications school in quantico for about a month then they go to pensacola for crypto school. My friend is a signals intel O but unfortunately marine intel officers like signals and ground intel who are sort of hybrids (ground intel officers also go to the infantry officer course), and because of that are frequently given the job of their more mundane side. I.e. my signals intelligence officer friend was in charge of a regular, non-intelligence communications platoon and my ground intel friend was put in charge of an infantry platoon (when they're officially supposed to be in charge of scout-sniper platoons). Which is fine, but they were expecting to do the top secret stuff, if they had wanted to work in the non-intel side they would have requested those MOSs instead. Of course being assigned a job you're not officially trained for happens all the time in the Marines.