If this actually worked, there would be boats sitting in the middle of the Atlantic and Pacific with computers on them. I have no idea if they are sitting there, but I'm pretty sure if it was an idea that worked, it would be done by now.
On the other hand there are probably cheaper ways to beat your competitors than crewing a boat, paying for fuel/electricity, havign a datacenter crew on board, dealing with the challenges of getting a high bandwidth signal hundred of miles off-shore, etc. Specifically you could invest in ever-faster custom processing in hardware or a lower latency connection will still make you faster than your competitors. You don't need to be as fast as possible, you only need to be a tiny bit faster than everyone else.
On the other hand, if someone can take advantage of economy of scale (by being a neutral provider to many brokers) and provide an advantage over everyone at a lower cost than the above options, then everyone would need to use it to compete effectively.
My guess for why people aren't doing it yet is that first, there are still untapped locations on land that are easier to get running (example: the Chicago to NY fiber run), and second getting the data to the ocean point isn't trivial.
I think if you want to look for early signaling of someone doing this, look for where telco folks route new fiber runs. If they take a longer path to go near one of the points in the paper I'd bet they're at least hedging for the possibility of using that fiber run for arbitrage.
Edit: a more mundane explanation may be that the arbitrage is so low margin that the payback time isn't practical vs the initial investment.
I don't think the cost/benefit is acceptable considering more serviceable locations that already have fiber, like Hawaii in the Pacific. Operational costs for a floating platform are much higher, and fixing anything major would take days instead of hours.
What about DDoS attacks (N/A if these are private network links) or real pirate attacks?
Notice I did NOT state the corollary, "I don't see trading boats in the Pacific, so it's not possible to do.". I suppose that could be accidently inferred from my statement, but that was not my intent.