In my opinion, there is becoming too much redundancy in the .gov, much less .mil. No offense meant in the least to our resident soldier/sailor, but why does the Army need it's own navy? Why does each branch of the federal government have its own law enforcement agency, like the Department of Education SWAT team? Seems to me that with some interagency/interservice cooperation (forced if necessary), a lot of tax dollars could be saved.
None taken, and I can actually answer that question. Without going in to really exhausting detail on US doctrine and service capabilities, the answer is that there isn't redundancy between the Navy, Army Watercraft, or, (even though you didn't mention it) Military Sealift Command. The Navy focuses on Combatant craft, and direct support craft for the Combatant's. What Landing Craft capability they have is limited, and embarked with the USMC Expeditionary Forces it supports. It is also tilted heavily towards contested, short term landings. (i.e. D-Day) LCAC's, AMTRACKS, and 1600 series landing craft are not self sustaining and have pretty short ranges and times between refueling. US Army Logistics Over The Shore (LOTS) capabilities is designed to sustain a force long term in uncontested, but austere environments. 2000 series LCU's and LSV's are designed to be self sustaining for weeks, and carry people, supplies and equipment from ports to remote bases, or from ocean going ships (MSC) to shore where there is no deepwater port or it's been damaged.
The capabilities are in separate services because of what the different watercraft missions support. The Army Watercraft support an Army mission, and it makes sense to have them organic to the Sustainment Brigades that will need to use them.
All of that to say that, yes, there is some redundancy in the .gov, but there are also often reasons things are done the way they are. The soundbites and memes that everyone loves often don't communicate the subtleties of disparate missions. Take the DOE SWAT team. Is it just using some of that sweet DHS funding? Maybe. Perhaps there is something specific about the warrants and crimes that DOE is using it on. Sure FBI HRT could learn a new skillset, but there is only so many training hours in the day. At some point it makes more sense to have a team dedicated to being very good at the one thing they do, rather than the FBI guys being sorta OK at a bunch of mission. Does anyone here actually know what DOE SWAT's mission parameters are?
So back to the OP: Should the USMC have their own Cyber branch? Probably, but a small limited one.
Much of the Cyber War environment can, and should, be handled by the Army and Navy Cyber commands with their resources, and the power and equipment on the ships and facilities. But as the battlespace evolves and weapons become more robotic there are real missions at the tactical level for cyber war. The USMC company advancing 150 miles inland from landing might very well want a guy in the back of one of those tracks with the knowledge and gear to hack and disable any enemy drones that pop up. Or someone that is monitoring the local EM spectrum to make sure no one is hacking and/or spoofing the Blue Force Trackers (or whatever the Marines end up using for that capability). Or folks at the Battalion level that can detect and DF low power encrypted bursts (like Enemy data networks). Stuff like that isn't well handled from hundreds of miles away by folks that don't speak Jarhead when info has to be passed quickly and clearly.