In his book "The Underwater Photographer" Martin Edge recommends to use Shutter priority ( if avoiding motion blur is important) or Aperture priority (if high depth of filed is important) in a situation you describe - you will probably shoot at different angles, with the sun, with side lighting and against the sun. He says that with fully manual camera settings there is a chance that you miss the one or other shot because you fiddle with your camera.
You might try to cross-change the cables to see if one of them is responsible - I personally had severe TTL problems with my EM5 I and a YS110a until I changed from no-name to a genuine S&S fiber optic cable. From then on TTL was fine.
A dealer at the Boot exhibition told me that from customer feedback some Oly cams seem to be very sensitive concerning TTL. He said that in many cases the cables are responsible. On his advice I tried the S&S cable - et voila!
You still will need your main strobe to trigger the remote one! (If you do not want that you will need a very long cable...) The remote strobe will have a cable to which the RSU is connected, The RSU needs to face your main strobe whereas the remote strobe can face any direction and/or can be hidden.
The triggerfish is/was a non-commercial DIY-solution. No idea about Seacam, but you could contact them.