I'm headed to the Galapagos next month and am really torn on whether to dive with a single Ikelite SS200 or two of them. I'm shooting a 5D with Canon glass 16-35 and/or a 14mm.
I enjoy shooting big pelagics with one SS200 and I like the resulting images. I wouldn't do that with any of the small digital strobes, but with a big gun like the SS200 I am able to light the subject and put the light where I want it. In galapagos you will almost certainly have the opportunity at salema schools as well as other smaller fish, so for me two strobes is necessary in that situation. I use a couple Z240's for double strobe and a SS200 for single strobe use, usually hand held. I have found the big pelagic schools very difficult to light fully so purposefully placed light from a single strobe renders images I enjoy more than blanket coverage from two strobes that falls short of complete coverage. For my eye a splash of light works better than a blanket of light that falls off at the edges and implies a vignetting (that's just my own personal perspective, not a lot of photographers agree with me

).
If you are concerned about heavy currents keep in mind that they vary from day to day and site to site. I've seen darwin when it had no current and have seen it when I could not get a strobe arm to stand up. So it might be advantageous to take two or three strobes and then use them in numbers or combinations you are comfortable with for the given conditions that day at that site. And then, of course, the redundancy consideration is applicable as well.
Have a great trip. I hope the fishes and conditions cooperate with you! 